They sign contracts.
But from what I've read, what they are told (vids will be sold to foreign markets and not uploaded online) and what they sign are different. In some cases they aren't given enough time to read the contract because the producers are hurrying it along and get annoyed if the girl wants to read every page.
It's a high pressure environment. Full grown Adults get talked into buying timeshares and cars they don't want and fall prey to scams every day from fast talkers that use effective high pressure tactics. Now imagine you're 18 with no money, nowhere to stay, no way home, in a room with a bunch of annoyed men telling you to sign a contract or else.
The difference between what they are told and what they sign is entirely their fault, as unfortunate as it is. Seriously, at 18 you know what is a contract.
However, being pressured to sign a contract without reading and understanding all the terms is criminal. Wouldn't that be enough to invalidate the contract?