Venice will charge tourists who donβt spend the night in the city a β¬5 entry fee
(english.elpais.com)
Tourist tax is a thing in many touristy places in Europe, you usually pay it in cash to the hotel or guesthouse owner and it's around β¬1 per person-night. So charging β¬5 for tourists not staying overnight is totally fine and it's weird they didn't do it before.