We don't really know about that. Streamers are being paid for doing their job.
To the extent of using their payments to join the wagering, I'm not sure if they are prohibited, because they are entitled to own the money
and if they choose to use that to gamble, it's their right decision to make not unless they are tied with any rules from the site.
Only the streamers or the team will have the perfect answer to this. We should enjoy though all those perks that are provided by Rollbit team.
Streamers also get paid from ads (not sponsorship, literally ads like the ones we see on TV) on twitch, and they get money from subscribers and donations as well. So when you see someone that gets 1000 people watching, that is not just one income, that is many many different incomes, hell even using certain equipment is sponsorship.
I knew a guy who was "gambling" in a game, sort of like cs:go knife whatever things, similar to that. He was getting paid from 380 or so subscribers which was close to 700-800 dollars of its own, then getting paid for using certain brand keyboard and other equipment, and getting paid for playing that gambling website for that game, and then getting paid donations on top of that, so in a month while subscribers were 700-800 dollars, he was getting maybe 3k+ dollars total. Streamers love their subscribers but believe me that is not most of their income.