I was recently building a web portal that could accept payments. I was looking into ways to avoid the extra credit card fees we'd have to pay to accept them. I was told I can't charge an additional "credit card fee" because a lot of states outlawed them, but was told I could just relabel that fee as a "convenience fee" and it would be completely legal.

Welcome to America. Fuck you.

The hotels receipts I've seen (I worked approving travel expenses, so I've seen a lot) they differentiate between specific locally mandated fees and their own fees (which is probably part of the legislation that compels them).

Then why do they vary by hotel?

You've got the right idea for the European system though. "Tourist tax". It's usually low enough that it's not a big deal, but I was shocked when I checked in at a hotel in Greece and the receptionist wanted 5€/night from me.