That'd be a stupid reason to outlaw BTC. They're used for so many different things, but I do agree that most of it in black market stuff. Don't try to argue this, just look up the deep-web and hidden wiki, that's only scratching the surface though.
I think that's why people are afraid that they might try. It's the stupidest thing to do, so it's the most likely thing the government would try.
A perfect example, ban guns and there will be no more killing.