But if you take a train you are susceptible to being taxed for entry to Venice, since they can just add some fee to the train fare, to either pay for maintenance of the city or simply to reduce numbers.
Kinda like a toll booth would make you susceptible to the same thing on the bridge.
The problem with both of those schemes is that there are a number of exceptions: people who live in the region, people who spend the night in the city... You don't want to place the burden of checking your hotel reservation on a train employee during a 5 minute ride or at a toll booth.
