Cheaters can be incompetent. I find the timing of trades of stocks closely related to what congress was voting more compelling than the under performance. Maybe you are right, but either way, the incentive should not exist at all - lawmakers having access to the markets while in office should be a blight of shame on a nation.
This is just another form of reporting an executive "suddenly sold a bunch of stock" (when they filed a 10b5-1 plan about it months ago).