Yes, because tumbling services aren't designed to launder money or anything...
Just using Bitcoin for transactions doesn't eliminate legal obligations. I imagine that if they ever got popular, tumbling services would be the first things that would be regulated (or shutdown by the gov't).
...except that anyone can set up a tumbling service in any political jurisdiction. It is not practically possible for the US to shut them down; the best they could do is try to police domestic users of tumbling services, which is a much harder problem.
It is also difficult to tell the difference between a tumlbler service, and some other various more legitimate uses.
The level of regulation it would take to actually trace lots of tiny transactions between pseudonymous parties is next to impossible. How is me paying a friend back for half the pizza we shared to a new address of his going to be handled?
Just using Bitcoin for transactions doesn't eliminate legal obligations. I imagine that if they ever got popular, tumbling services would be the first things that would be regulated (or shutdown by the gov't).