I think the thing is insider trading genuinely
is exactly part of how prediction markets contribute something that was not offered before: a more accurate way for the public to gauge the probability of future outcomes. I, as a rationalist, think there is a lot of good that results from that.
It just happens to almost always be the case that the bad greatly outweighs the good, and it usually just leads to things like brazen war profiteering (+ plan-leaking that jeopardizes operations, in the event the military operations were perhaps for a morally net good cause).