PokerStars recently announced a deal with New Jersey to allow players from that state to play, though they'll be playing only against other NJ players and not from the global pool of PokerStars players. At least it's a start. A restart.
Officially PokerStars currently does not allow players from the U.S. At some point in the distant past, I remember seeing a message on their site indicating that if you try to trick them by using VPNs or similar and they find out, they'll reserve the right to take your funds and ban your account. I can't find a similar message at the moment, though.
My guess is that some people might be skirting the law, but that the people who are doing so would not likely be so bold as to broadcast that fact on twitch. A lot of American online pros have relocated to Canada or Mexico or other online-friendly locales, so you might have found some expats to watch.