Most people don't even read the Terms of Use, and it's pretty difficult to find this if you didn't read it when you signed up, but under this site's Terms of Use, Section 5, subsection (C), it states:
By posting Content on any public area of RoosterTeeth.com, you automatically grant as well as represent and warrant that you have the right to grant to RoosterTeeth.com, an irrevocable, perpetual, non-exclusive, free of charge, worldwide license to use, copy, perform, display, and distribute such information and content to RoosterTeeth.com and that RoosterTeeth.com has the right to prepare derivative works of, or incorporate into other works, such information and content without liability or payment to you.
Loosely translated, that means that by posting anything to this forum, you're giving the right to Rooster Teeth to use it for free, however they want, forever. They don't even have to acknowledge that it came from you in order to use it.
Don't like it? Only two things you can do-- (1) Don't post anything at all, or (2) Only refer to your material in a link. This agreement does not give them any rights over anything that isn't physically posted on their public areas. (Technically, they'd have the right to distribute the link but not whatever content is actually linked to.)
Now, with that said, here's why they have to do that...
Intellectual property rights are a tricky thing, especially in widely-distributed artistic creations like web series. The RT staff do read the forums, so they are exposed to a lot of ideas. If a user said, "Hey! I think a talking bomb/purple Medic/guy named Donut would be really cool to include in RvB," and it showed up in a later episode, that person could make a case that the idea was stolen and used without credit. Especially since RT is selling their DVDs, the person could conceivably demand a share of the income. Coincidences happen, and Burnie could have written that idea into the season way before the poster ever posted it, but since there would be public evidence that someone mentioned it before the episode aired, Burnie would have to prove he came up with it first.
Unless they put in a legal claim that ALL submissions become their property to use as they see fit. That way, they're covered, no matter what the posters suggest, even if they use it. It's not really meant to be used to rip anyone off--but you do need ro realize that they absolutely have that right, now that you posted it (and you can't become a member unless you say you agree to the Terms of Use--including this one.)
If you don't want to give up any rights to anything you post, post it on your own website and link to it, if you must. You never know when the RT Crew could suffer collective insanity and publish the entire forum text in a book, but if they do, don't bitch that you don't get a dime off of them.
Here's a link to what you signed away when you joined: Terms of Use
They'd have to get up pretty early in the morning to pull the wool over the eyes of a contract this air-tight.
Good thing, too, because Rooster Teeth totally stole that one idea the 12-year-old and his buddies came up with about making there be a "person" with a "funny voice" in their movie. Those guys are total scam artists.
It's not the first group I've been a part of that had to do this, just to cover their own butts. It's why movie studios will return unsolicited scripts unopened and unread, just so they aren't exposed to lawsuits of this nature.
Just a point to reinforce what Sev said about coincidences, I once submitted a short story for publication and had it returned with an apology because certain plot elements were too similar to another one of their stories. Now this story hadn't even been published yet, so I know it was simply a coincidence.
These things happen, I doubt RT actually plans to use it to steal someone's work.
Though I am curious as to what even caused this thread to be made....
The "mail it to yourself" thing does not stand up in court. Besides, even if you have copyrighted something yourself, posting it here gives RT the right to use it or make derivative works from it without giving you any money for it, nor even credit for the idea.
I doubt too that anyone from RT would try to steal our works. However, those publishing short stories should know that posting stories on this site constitutes a form of publication, and most publishing houses won't work with previously published material. Something else to think about.