I'm not estimating organized crime at all, because I'm very ignorant of it. I'm only saying that I would expect the potential bettors in that prediction market to make some estimation of how difficult manipulating the market would be, and not many people would bet on a market they deemed to be easily manipulable. Another mechanism that could exist is that the organizer of the prediction market could itself judge whether a particular market was manipulated and have policies for what to do in such a situation.

But in reality, that just doesn't and didn't historically happen in stock markets. Buyers didn't accurately estimate the likelihood of manipulation. The government had to step in.