These gambling sites have always been really shady.
The reason why there's gambling site ads during tournaments is because the leagues have no other choice.
With the rise of gambling sites leagues started noticing that streams have a lot less viewers if there are no bets up for the games. So they had to start making deals.
So leagues advertise these sites now and in return bets are opened for their games. And now the leagues are stuck with them.
"What's that? You don't like our questionably legal methods. Okay we're canceling all bets on games from your league which makes you lose half of your viewers. Come back when you're willing to make us more money."