They might also know the advantages of it. But they saw many people in their country is commonly using it in an illegal way so they have no choice but to ban it..They ban it because it will just harm more people if they will continue using it. The adoption of bitcoin on that country is not good.
I agree with you.
The fact that bitcoin can not be tracked and the fact that it is impossible to idenity a person from a bitcoin address makes it pretty easy for people to maintanance scam websites and to buy drugs and etc.
That's one of the reasons that bitcoin users are afraid that the governements will ban it.