>Prediction markets don't have any "natural" reason like that for excluding insider trading. It's just "game designers" crying their hearts out when someone ruins their game by having an advantage.

I can think of a few very good reasons you would want to prohibit insider trading on prediction markets. Betting on war outcomes; being incentivised to commit war crimes or throw vital operational goals for financial gain. Wagering on public figures' jobs; being incentivised to harm them.