It's hard to prevent cheaters on online chess, at least on short run. But as far as I know, there aren't perfect crimes and sooner or later users using third party services to compete online are going to be busted and have their access to the platforms restricted (at least serious platforms).
Well in the first place cheaters are simply people who are actually testing the computational/processing ability of their program (most of the time anw). I mean, I certainly wouldn't waste time trying to run a program that's pretty intensive (since it tries to simulate most of the possible moves) to win an online match. For testing purposes, it may be possible, but I highly doubt people would do it for the money cause after all, if they were able to test something like that, they'd probably be better off doing something else to make money.
Plus in the first place, I don't think betting and chess go hand in hand? It's why most tourneys at the very least are done personally, or with hand cams and whatnot. Maybe betting as in sports betting, but that's about it imo.
There is no problem in using or programming bots for hobby, testing and learning purposes, but it can't be done in competitive matches that will count for increasing players' classification in a global ranking, or to earn money against others during tournaments.
I don't know how often this kind of thing happens, but from humans, of course you can expect cheating anytime and anywhere. Sometimes it's not even for the money, but for the status.