I think a common argument is that it is to offset externalities but in reality I've never seen a sin tax that is based on the cost of the externality. The tax

I think a common argument is that it is to offset externalities but in reality I've never seen a sin tax that is based on the cost of the externality. The tax on cigarettes is not pegged to the expenditures on COPD and other diseases and etc. Afaik, all sin taxes go into the general coffers so they can't really be based on externalities. Plus the fuel tax doesn't even pay enough for road repair never mind any externalities (especially considering there's often no sales tax on fuel).