Faulty argument. Just because the government doesn't issue it doesn't mean they can't control or police it in other aspects. Drugs like cocaine and heroin aren't linked or controlled by a government institution but they're still illegal and they police and punish their usage and distribution. Governments can't fully stop people from using bitcoin, but they can certainly regulate it and put restrictions in place that would hinder its use or even criminalize it and make it illegal. I don't know under what grounds but if they wanted to make life difficult for us they certainly could. I'm sure they could put some sort of ban on it being used as a currency or even prohibit people from using it as a speculative asset, but like I said, 'bans' likely wont come under the guise of illegality but just regulating it to death which will put off both investors and merchants alike from getting involved. Hopefully they won't and they'll see the benefits of allowing it themselves and the tax money they'd be missing out on, but they don't really see that with the legalization of drugs so who knows what their thinking or rationale would be for banning or regulating bitcoin in whatever capacity.