Um, this is wrong. The effect of insurance on behavior is called "moral hazard", and it is a market imperfection. People who contract for insurance will be best off if they minimize resulting changes in behavior, and insurance contracts with severe moral hazard issues are simply not traded. (For instance, unemployment insurance is not traded on the private market.)
That's only true for cases where the insured has significant influence over whether the insured event happens. That's not the case for legislation, unless you're an influential legislator or lobbyist; and, to the extent that moral hazards cause a problem there, it still works to make the legislature impotent (since they can no longer trust each other not to use their votes to game the prediction market).
And I think you've significantly overstated the impact of moral hazard problems: there are numerous instances of insurance contracts where the insured has...