Bitcoin is threat for current system. Some government's think just about making profit on peoples back, with many taxes high prices they hold their power, and that is how they rule the country. They already know how to control the supply of money in the state, with that they control everything, can you imagine what is bitcoin for them now?
Countries that think about future, and about people are open for bitcoin because bitcoin is future, others that do not support bitcoin are the countries where leader's think just about their well-being.
in legitimate transactions only. If they cannot regulate it, that is the time to ban it, but I'm sure they can regulate it.
There is a profound wisdom
And it says that you should never forbid what you lack the power to prevent. It is attributed to Napoleon Bonaparte, and I guess he certainly knew what he was talking about (as we would say nowadays). If they ban Bitcoin but, nevertheless, still can't prevent people from using it, that will backfire heavily since no one will be giving a fuck in the future about what this particular government says. The only viable way for governments to fight with Bitcoin is to kill it from inside by bribing corrupt miners so that they either prevent any improvements the lack of which will make Bitcoin obsolete over time, or deliver a direct attack against Bitcoin (after they grab enough hash power)