I don't believe that those winnings are paid by the casino intentionally and these guys are using a specific program provided by the gambling site to increase the probability of winning. Since they are partnered with the casino, they will be given a huge bankroll to play with. In gambling, the longer your bankroll lasts, the more chances to win big and that's the reason why paid streamers always win big in their stream.
I will accept the bankroll distribution, it's an open secret and this is to test the platform and entice gamblers to deposit more and play the games like streamers do, it's to entice gamblers that they can duplicate the streamer's success.
I guess streamer can't stuff like blackmailing, exposing, or anything like this as they are most probably has an agreement with the casino before they engage with each other's terms. Unless there's a bit of a misunderstanding and the streamer resort to that, It will spark a legal disputes.
But if a streamer is well-paid, we'll expect no negative feed backs from them on that specific casino they are working with.
It's not even an open secret, It's a common sense for everyone that If someone is working for any gambling platform it is expected that they will tweak their system a little bit so the person who is advertising or playing live on the platform would look like he's winning everytime. And yeah to entice people to play on that specific gambling platform.