The reason is people take it as given if you mess yourself up the rest of us will have to pay for your treatment. If I have to pay for your mistakes, then I want some say in what you're doing.
Personally, I'd rather live in a more libertarian society, where you could do whatever you want (to yourself) and accept the consequences. But that's not the society we live in.
But alcohol and tobacco are just as harmful in the sense of having to pay for the treatments and the damage to others as the rest of the drugs. Why ignore them?
There are all sorts of restrictions on alcohol and tobacco. In the US alcohol would be illegal if the government hadn't tried and failed to make it illegel.
Personally, I'd rather live in a more libertarian society, where you could do whatever you want (to yourself) and accept the consequences. But that's not the society we live in.