While not illegal here the states I do notice that even recent media reports about Bitcoin talk about it being used for illegal purposes. Lately this seems to be the news :
Bitcoin hit a new record high
Bitcoin is a digital currency that can be used for anonymous drug purchasing.
Forget all the regular day to day stuff the bulk of bitcoin transactions account for, let's focus on the few instances of bad.
I think that buying illegal goods with the help of bitcoin, is not a reason to prohibit crypto currency. Drugs and dollars are buying, but the dollar today is very well in the currency market.