Best case scenario for Bitcoin is that MtGox is working with US feds to identify SR vendors, and announce a mass bust of them. This would mean that the feds are working _with_ the exchanges to identify lawbreakers, rather than trying to shut down the whole damn thing like they did with SR. No one likes the strict MtGox AML rules, but that might be the difference between the exchanges getting shut down LR style, and being allowed to exist by the Feds.
Why would the US government want to crack down on SR?
I don't think they care too much about who's smoking what. What they do care about is their own power. People leaving the dollar is a threat to their power, so they are going to try and make it hard for people to leave the dollar. Terrorism & drugs are actually good for those in power, since they can profit from fighting them. So, no, the exchanges are gonna be targeted anyway, compliance or no compliance.
The good thing is, they probably can't do too much about the exchanges in Russia / China / Argentina / other areas not quite entirely under their control. Another good thing is, they have other issues apart from fighting Bitcoin, and any new war is going to speed up the incoming collapse.