Exactly right. I will to do what I can to help the police catch scammers and crooks who want to steal from people. The police might use those same tools and techniques to help catch people who use bitcoin to pay for drugs; I can't stop them from doing that.
Are you saying you would want to work on "coin un-mixers", software that attempts to reduce the effectiveness of coin mixers? (Using statistical analyses on the public data of the block chain.)
He's not saying he wants to explicitly work in that field. He'll just explain bitcoin to interested parties (a good thing, since they get the real story, not the ZOMG think of the children version) and explain how the protocol works. If the FBI decided to use their knowledge of bitcoin from this to make such a "coin un-mixer" then that isn't Gavin's fault: he merely gave the gift of knowledge, much as many of you forum posters did to everyone here who came in asking questions and needing help understanding bitcoin.