Match fixing happens regularly, so regularly that we have the term "match fixing" with a Wikipedia article dedicated to it. The presumption that match fixing doesn't happen in practice is unfounded.
It's a question of scale. A few horses aren't a big deal, and both sides (the horse racing, and the bet) are heavily constrained in scope. Both sides become far less constrained with prediction markets.
Maybe if we were objective, horse betting should be shut down -- but we kinda don't care.
