Well markets give a huge financial incentive for it. Before you had to get paid a bribe for an intelligence agent. Now you can just "legitimately" bet on a market. It's a LOT easier and more spread out, I imagine.

The existence of banks gives a huge financial incentive to rob them. That doesn't mean we should get rid of banks. It means we should create a huge disincentive to rob them (which we do). Same thing needs to happen to people using national intelligence secrets in prediction markets.