They don't guarantee corruption, because if the corruption shows up on the prediction market it has occurred (or will occur) elsewhere already, and if large amounts of money are betting on a certain outcome, others will follow, and thus the payout won't be as large.
Surely such prediction markets are a guarantee of corruption?
That or I just don't understand - if a market exists that takes bets on human actions such as passing laws, how long someone will live etc, the betters will have a very active interest in doing everything they can to make a large profit, of which corruption alone is the most obvious target.
I mean in a way you're right - it is wisdom of crowds, albeit crowds of people that "know" only because they're corrupted the final event.