Clearly, it's going to happen anyway at least some of the time, but the "what's to stop" is pretty obviously getting caught. It's already happened in at least three instances from the past year I can think of, though I don't remember the names because they were all pretty small-time players (Jontay Porter, who was already mentioned, is one of them). Anyone that is caught doing it gets instantly banned from the sport for life, which hopefully provides an even larger counterincentive compared to whatever incentive there is to attempt throwing a game.

It's not perfect, but the leagues don't have much of a choice. The US Supreme Court ruled that banning sports betting was unconstitutional, so until the court rolls over to new justices with new opinions on the matter in a few decades, it isn't going anywhere. It doesn't make any difference how much some fans on the Internet don't like it. Federal lawmakers can't ban it.