>What if someone (could be an individual or even an uncoordinated group) bets millions of dollars on you not doing X, in the hopes of you taking the opposite bet and doing X?

If the market is efficient and aware of the bribery effect, others will bet that you will do X up to the point where the indirect bribe is equal to the cost of you doing X. If you have private knowledge that a bribe would be taken, you probably have the access to do it far cheaper off market (and you can still use crypto).