OK, my own disclaimer first. I'm not a lawyer, either. However, I did have to research copyright owners and seek permissions for works as part of my job a number of years back, and thus I learned a fair bit about intellectual property (and the difference between copyright and trademark, which often get confused, although that's neither here nor there).
Fact is, parodies often don't require the permission of the rights holder, but it's likewise a fact that folks like Weird Al Yankovic nearly always get such permission before going forward with releasing a parody commercially (the exceptions in his case tend to be miscommunication rather than negligence, FWIW). The issue here is what's called "Fair Use Doctrine."
The trick about using "Fair Use Doctrine" as a defense is the reality that anyone can file a lawsuit claiming infringement whenever they want (even if it's without merit). It's only once a suit is filed that the courts look at a (purportedly) derivative work and apply the rules of "Fair Use Doctrine" to determine whether the derivative work is covered (or not) by it. So, something that a writer may well have thought was a parody may later be determined by the courts not to be a parody, at which point the derivative work could find itself in hot water.
To apply this to Transformers, for most of what we're talking about, the competitors have never actually (yet) filed a complaint against the Transformer they might claim is a derivative work. We're usually just looking at cases where Hasbro's lawyers are playing things exceptionally safe, so as to avoid having to deal with any lawsuits in the first place (even ones they might win). A notable exception to this is the case of Jetfire, which as many here know, was originally created using a mold that is associated with the Macross line (whose American equivalent, Robotech, is owned by Harmony Gold). Harmony Gold tried to sue Hasbro for a repaint (with new accessories) of a GI Joe Skystriker as Jetfire for an SDCC exclusive. The courts found this accusation SO out of line that they dismissed the case with prejudice, freeing Hasbro up to use Jetfire without fear of another such suit in the future.
