Yes, you are right. People are always able to buy and sell bitcoin. They can trade it peer to peer. But some governments are trying to make it difficult for the people specially when they want to convert bitcoin to their national money or their national money to bitcoin.
Any way, governments cannot stop people completely.
Governments can't do anything if the exchange has been made in the form of p2p. You don't convert your bitcoin to fiat, you exchange them. It would be hard to figure out how the exchanges happened for a normal bank. Only thing is that banks can't deal with those who offer crypto services.