Well, effectively outlawing tumblers involves investigating people that pay with tumbler-tainted coins. I mean it more in a Patriot Act type legislation sense rather than probable cause [0]. Obviously, if you've got non-tumbled coins with a direct criminal taint then I'm pretty sure that's probable cause already. Just to be clear - coins are not forever tainted in such context. i.e. someone extorts 1 BTC, the victim reports it - it's now tainted. The criminal pays the 1 BTC for pizza, police arrests him. The coin owned by the pizza place is a clean coin again now.

[0] tho I guess in some cases it could very well fulfill it (>50% probability of a crime having been committed)