Because most of those interstate travelers provide some benefit to the state itself, whether it's stopping for gas, buying lunch or staying at a hotel, all usually involving some sort of sales or excise tax.
Maybe in California, but not in some of these tiny Northeastern states. Come to think of it, that’s one reason why tolls are so common up there. It’s the only way to get revenue from someone who is driving through the state without stopping.
There really is NO equitable system in my view, for instance in South Carolina the sales tax on grocery purchases is two percent versus eight percent on most other consumer goods. On the other hand the sales tax on all restaurant meals, including fast food sandwiches, is TEN percent. The bright idea was that tourists would pay this tax and therefore benefit the state. The problem is that most of those people eating in the restaurants are local so Joe Smo who works for peanuts pays more in taxes for his burger and fries than the whole cost with tax included used to be when I was a young man. Taxing agencies are only interested in raising taxes, they don’t give a damn whether the taxpayer is being bled dry.