Legal and licensed online casinos actually don’t directly control payouts in their slot games, although they do exercise some control. Casinos get to choose what games they want to offer. That means they can decide exactly what slots, what table games, and what other types of gambling games they will feature.
Slots developers adjust the RTP % when creating the games, and gaming regulators also set legal requirements on those payout percentages. That means in jurisdictions where real money online casinos are legal, you shouldn’t encounter slots with ridiculously low RTP percentages. Of course, you will find some slots have higher percentages than others.
Understand, though, that an online casino cannot go in and change the RTP percentage for a particular game on its own. That can only be done in cooperation with both the game developers and gaming commissions that regulate the games.