I asked my lawyer friend (in your defense, he is a patent lawyer, so this scenario is not his forte) about the legality of this situation.
- He says that it is unlikely that anyone would be able to legally action Twitch because of the clause in the user agreement (which I quoted a few pages back). He says that the very first obstacle to actioning Twitch would be to challenge the validity of that clause--an unlikely feat.
- He says that Twitch is not an employer. He says that what Twitch and its partners have is "more like a fee-sharing agreement" (which is completely supported here:
http://www.twitch.tv/p/partners).
- He says that Twitch and sites like it have "broad powers" that enable them to defend themselves easily. He says that any challengers are "most likely out of luck", and then he points out that this is not his area of practice, so he can't say for sure.
So, unsurprisingly, it's not a good idea to rely on Twitch and consider it your source of employment. That said, I fully respect and support anyone who makes a successful income out of this kind of thing; we live in a very competitive and industrialized world and I think it's wonderful that young people are coming up with new, positive sources of income.