Well so fiat money is also used for illegal transactions but the government does not prohibit fiat money. 
Criminals using bitcoin for illegal transactions is reasonable because if using bitcoin will be difficult to track who they are, bitcoin is very anonymously.
Criminals using bitcoin for illegal transactions is reasonable because if using bitcoin will be difficult to track who they are, bitcoin is very anonymously.
In US, more people die from car accidents than from exploding hand grenades.
Can you guess why live hand grenades are largely illegal to own, while cars are not?
Tell me if you need a hint.