The answer is simple - you price in the chance that an insider could have an edge on you when making your prediction. It's all part of the game.
We also care about the world outside the prediction markets. If decision-makers have an incentive to "throw the match" by making surprising decisions, that will impair the decision-making, which will affect the rest of us. Likewise, sufficiently-wealthy actors will be able to manipulate the behaviour of officials by betting against the behaviour they want to see, much like an assassination market.
Right, it becomes regular bribery with middleman and a funny hat.
"I didn't pay the judge to dismiss the case against me, I just hedged by betting I'd be convicted and he just happened to be betting I'd go free, and now my money is coincidentally in his pocket.