I think there is a difference between conducting a simple business transaction with a suspect individual or government (ex: I give you 1,000 licenses of MS Office and you pay me $500,000) versus facilitating whatever unethical behaviour the suspect is involved in (ex: I help build your gas chamber and you pay me $500,000).

Should a restraunt owner refuse to serve an order of eggs and bacon to a customer, because he is suspected of being a drug dealer? I think not.

Should a realestate investor back out of a deal, because he discovers one of the other partners launders money for drug dealers? I think that's prudent, and it's his perogative.