Shouldn't be illegal.
> exchanging it for dollars
Shouldn't be illegal.
> You give banknotes to someone in exchange for a token which you don’t know how it was obtained.
It doesn't matter how it was obtained.
> In most cases, it is facilitating drug exchange and hitmen
Oh... Just like USD then.
Why don't you go after them for those crimes? Without coopting us into your warrantless global surveillance network?
> unless one can prove where a Bitcoin comes from
That it is even possible to prove that is actually a failure of bitcoin's design. Thankfully Monero fixed it. Money should be fungible. There should be no dirty/clean bit attached to coins.