Because the knock on effects aren't just in the tourists not spending money, but clogging the streets (go to central Amsterdam on a Saturday evening sometime, it's insane) causing trouble, and even if there's money that's not going to the cooks, servers, museum docents etc.

There is likely still a price at which point the strain would be reduced enough that the taxes levied on those left would be worth it if e.g. used to improve local services or reduce local residential services fees or taxes.

But maybe it's not worth the hassle to figure out for some cities.

The port could be owned by the state or some private entity, not the city.

I think e.g. the Venice port is owned by the state.

Government can always add levies if it wants.