I think this is a good idea, as Venice has lots of issues caused by the excess of tourists (i.e.ground erosion), however they should also greatly limit cruise ships too (they say they want to do it from time to time, then do not follow through).
At the same time, I am a bit perplexed. Tickets are enforced with random checks, and if you do not have your ticket you will need to pay a fine. I am not sure that you need to pay on the spot tho. If that is the case and you have 50 people from - say - the US or China, you can just avoid paying. Who will come looking for you anyway? Yes, they can kick you out, but again this won't work if they find loads of people without tickets.
If it's anything like speeding tickets in most of Europe your place of residence determines whether you have to pay on the spot or not. For example Netherlands and Germany (and many others) have an agreement in place where your home country collects the fines so you don't have to pay on the spot. But if you're from a non-EU country you'll almost certainly have to pay on the spot because there would otherwise be no way to collect the money if you refused later.
At the same time, I am a bit perplexed. Tickets are enforced with random checks, and if you do not have your ticket you will need to pay a fine. I am not sure that you need to pay on the spot tho. If that is the case and you have 50 people from - say - the US or China, you can just avoid paying. Who will come looking for you anyway? Yes, they can kick you out, but again this won't work if they find loads of people without tickets.