You clearly never look at your hotel room receipt. If they charge you (which they almost always do, I am yet to stay in the hotel that doesn't charge it) they have to state the value per person, per night. On your receipt it would be called "City Tax".
It is a bit like environmental charge at the dealers whenever they service your bike, change tyres etc.
It is essentially waste management fee (council tax?), which hotel does pass on to you. At least that is my understanding.