Okay, let's get stupid here.
If the fans are free to assemble any sort of canon they would like, it isn't a canon, just personal preference. The two words contradict each other, canon =/= personal preference. It makes no sense, but is really one of the few choices left to A/P/AvP fans.
So since we're talking about canon, as per the above definition of being something that is defined by events and facts within a story rather than through the interpretations of the audience over several disconnected and possibly contradicting stories, we have to look at what has been produced and go from there.
The movies are the source material for all EU, in this case.
The obvious must now again be stated, because it isn't already obvious enough:
In most EU, toys, video games, and comics (to a point, especially recently) are targeted towards children and considered disposable. Now, if we can safely put the insanity of including all of that crap in a movie's canon to the side, we can concentrate on EU material that isn't directed at children, like novels and novelizations.
(Note for clarity - You'll notice that the three above categories, toys, video games, comics, are often simply understood to be extensions/marketing of a product instead of canon-establishing story extensions to the product (like a film sequel.) Most toys don't tell a story related to the movie, comics are all over the place, and video games have never been so advanced that they have the capability to tell a story via the game that could have the clout and presentation to match a film they were based from until recently. Colonial Marines will have a big impact on this area.
Basically, there's no regulation because the media industry understands what these things are: disposable. They don't effect the movies, or the canon, unless the studio comes out and says they do, so they don't have to worry about cleaning up any messes, just making money.
Only in dark superfan dungeons on the internet do these issues arise where common sense is simply abandoned in a desperate argument to make sense of a mess.
I write those things off, even if I like them, because they were never intended to establish film-level canon. That's just common sense, and if you disagree or don't get it, stop reading now.)
Moving on, we have a wide array of novels and even novelisations that can expand a story quite a bit. As far as these things establishing canon, It follows suit they're licensing agreements like the rest and dismissed by studios with the aforementioned three other EU categories, but since they are traditionally a more adult media, it's excusable to want to include them as part of an R-rated film canon. They can offer some great efforts that expand a property...
Some real answers could be found in Fox's licensing agreements to companies like Dark Horse, which I don't know about. The basics of getting a license for an intellectual property are so that an outside company may produce and market that property without infringing on copyright and trademark laws.
If Fox authorizes licensors to establish canon, then there it is, and we're all stuck dealing with this shit storm mess. If not, then it's marketing to create products in the same universe (that may or may not be consistent) with no bearing on the film universe at all.
I mean there's a lot to that. If Dark Horse f-ed up canon so bad, why renew their license? Probably because it makes money!
Xenomprh, not replying to you because -
A) You believe that a sequel that expands on an original film is a retcon because no one saw the new stuff in the first film.
B) You get emotional because I bother you.