Copyright
A copyright is the (usually exclusive) right to copy, perform and distribute a work that is an original expression of a idea for a limited period of time. The owner of a copyright can be an individual, a collective or a corporation. The default owner of the copyright is the creator of the work; however, under United States law, copyright can be transferred or, if the creator was creating his work within the boundaries of a work-for-hire contract, belongs to his employer depending on the exact conditions of the contract.
In the context of the Transformers brand, Hasbro and TakaraTomy are the main copyright holders for their respective markets, and they often even act as representatives of the other one in their markets. For example, an American who violates the copyright of something that technically belongs to TakaraTomy will have to deal with Hasbro, who will act on TakaraTomy's behalf. Since most official Transformers-related works are specifically created under work-for-hire conditions, there are very few relevant things Hasbro and/or Takara don't hold the copyrights to, and they will try to amend that as well.
A copyright is not the same thing as a trademark. Since copyrights affect original expression of ideas, it is generally impossible to "copyright" a name, term or slogan, otherwise the simple act of "copying" (i.e. writing it down in a public venue, such as an internet message board) or "performing" it (i.e. saying it aloud in public) would amount to copyright infringement. Names, terms and slogans are protected as trademarks, which are only relevant in commercial contexts. Graphics and logos can be protected as both trademarks and copyrights. Unlike trademark infringement, copyright infringement can be committed by anyone; however, unlike trademark infringement, where the owner has to act as he might otherwise lose his trademark, it is up the owner of a copyright to decide whether he wants to pursue a particular instance of infringement or not. Compared to other companies, Hasbro and TakaraTomy have traditionally shown to be very lenient in this regard when it comes to taking legal actions against their own fans.
Copyright history
Modern copyright basics
Any time an original work is created that is an expression of one or more ideas, it is automatically protected by copyright. Ideas themselves cannot be copyrighted; for example, the basic concept "a robot that transforms converts into something else" cannot be copyrighted by itself, neither in terms of toys (anyone can create and release their own transforming toy robots without necessarily committing copyright infringement) nor fiction (anyone can write and publish a story that includes transforming robots without necessarily committing copyright infringement). It is the specific expression of this idea that will be covered by copyright. The limitation for whether something is specific enough to count as aon original work covered by copyright or not is known as the threshold of originality'. For example, Generation 1 Optimus Prime's toy is protected by copyright, as is the backstory of the Transformers lore as originally developed by Marvel (alien robots from the planet Cybertron) and the specific stories told in comics, cartoons, video games and movies. What matters are not specific details by themselves, but the work as a whole. The more similar someone else's work is to these specific works, the more likely he will be found guilty of copyright infringement. The more vague the similarities are, the less likely he will be convicted. (It's not really possible to quantify the similarities in percents, though.)
The default owner of a copyright is the creator of the work. If a work is created as a group effort by several people, they share the copyright to the work, or hold the copyright to certain parts of it. For example, if Jim composes a song and Bill writes lyrics to go along with the tune, Jim owns the copyright to the melody (which includes the right to reproduce written notes!), whereas Jim owns the copyright to the lyrics. If they shared the workload for one or two steps, they share the copyright and need to reach an agreement when it comes to releasing or licensing their shared work. It is also possible to create a work under a work-for-hire contract for a company, which is the default in the American entertainment industry (creator-owned works are the exception rather than the rule). Anything created under the conditions of that contract is copyrighted to the company. When Hasbro assigned Marvel to develop a backstory for the then upcoming new Transformers series, Marvel was working under a work-for-hire contract for Hasbro, and Marvel's employees were working under a work-for-hire contract for Marvel. That means any stories, characters and concepts they created within the boundaries of their contracts were automatically copyrighted to Hasbro, not to Marvel or the individual creaturs. Marvel tried to circumvent that eventuality for a few characters due to a loophole in the contract, however (see below for more details).
Any unauthorized use of a work that is protected by copyright constitutes copyright infringement, and can result in legal prosecution. A common misconception is that "anything that can be found on the internet is free". Any images found on the internet are by default protected by copyright, and therefore cannot legally be copied and republished without permission. Exceptions are works that are in the public domain, either because the copyright term has expired, the work has been created by an employee of the United States government and military within the boundaries of the work (those are in the public domain by default), or because the creator has deliberately released his work into the public domain. There are also specific licenses which allow individuals to use someone else's work without specific permission as long as the conditions of the license are respected (e.g. identifying the creator by name, non-commercial use only, etc.). However, even though any unauthorized republication of a promotional image depicting Optimus Prime constitutes a copyright violation (which means pretty much any internet fansite is full of copyright violations), it is ultimately up to the copyright holder to decide whether he wants to legally pursue a specific instance of copyright violation or not. Unlike trademarks, looking the other way and pretending not to notice a copyright violation does not put the owner in danger of losing his intellectual property.
Hasbro and TakaraTomy have a long history of being very lenient when it comes to the fans of their brand. The oweners of Asterix are known to be particularly rigid when it comes to defending their intellectual property (especially in Europe, where copyright law is even more rigid than in the USA in many regards). Compared to that, Hasbro and TakaraTomy's approach appears to be based on several questions: Is the copyright violator acting in bad faith? Does the copyright violator make a profit? Does the copyright violation reasonably cause actual (instead of just purely hypothetical) damage? Does the averted damage outweigh the the efforts and expenses for legal measures? Do legal measures hurt the public image by appearing as an evil corporation that takes its own customers to court? Note that there is a large leeway for copyright holders to operate in, and Hasbro and TakaraTomy have simply opted for the fan-friendly end of the spectrum. They have done so by their own choice and are free to change their approach whenever they want. For example, in the early 2000s, many fan sites were hosting scans of the Generation 1 Marvel comics and encodes of the Sunbow cartoon. Neither were officially being distributed back then, and Hasbro concluded that having those contents on the internet might help keep the interest for the Transformers brand alive, thereby having a positive effect on their toy sales. However, with the onset of a general 1980s nostalgia wave and an increasing interest in Transformers in general, Titan started releasing collected editions of the Marvel comics, and Kid Rhino released the cartoon on DVD. With financial interests at stake now, coupled with the fact that the owners of these websites had started to ask for donations to cover their bandwidth fees, Hasbro decided it was time to intervene and sent out cease and desist orders, a comparably harmless legal measure. The sites in question complied, prompting Hasbro to continue tolerating their existence and other, comparably minor (and not business-endangering) copyright violations.
