You are not wrong with your math but its about the security and laundering stuff that may come up front. Most of the countries are afraid that huge amount of money might be laundered with the help of bitcoin and thus it may cause the disturbance to economic circulation in that way. It has recently discovered that many cases of mafia has been found who are actually using the bitcoin this way only. This is the biggest and prime reason why countries are trying to avoid the unwanted use. They also want to protect the peoples funds by stealing methods, crypto fake exchangers and schemes etc.
Well, the security and laundering stuff is not an issue that cannot be solved and this is why regulations can be in place with the government being in control of the exchanges which could mainly serve as the main source where laundering can simply occur.
It is just simply because they are looking for every way not to make it work and reason why they would rather stop it than try to do anything about it at all. Unfortunately for them, it is a battle they are really not going to end up winning.