But the point is this: you CAN get permission if you go through the channels which we all know exist. I do not think it has anything to do with security at all; I think it has everything to do with making some money for the city and maximising all possible opportunities to do so. If that were not the case, then few movies would ever get made!
Ciao - Rob C
There are at least two basic reasons why they don't want you to open a tripod in the road:
Certain local councils have exploited the opportunity the Law gives them to tax people if they want to take pictures with a tripod. Rome is among those places. It is as if the entire town was a museum. You want to take pictures? You pay.
You have to go to a specific local council office, compile a form whereby you state where, when, for how long and for which purpose are you going to take the pictures. They will charge you a very small amount (such as 10 euros) and away you go with your permission.
This is obviously something nobody does: how can I know how the light will be, where will I be etc. Most photographers do not plan that way. Also, you spend 10 euros but you will have wasted a morning just in getting the permission.
If it is a "vigile urbano" (local council policemen) to stop you, that will be for that reason.
A policemen (state policemen) normally would not care at all whether you have a permission or not, but the problem is, whereever you go in Rome, that is a sensible place.
If it is not a public office it is an embassy, an ambassador residence, an airline office, a judge house, a MP pied-à-terre, whatever. And - Jesus - policemen are just about everywhere when you don't need them!
I am going to buy one of those Manfrotto tripod with a fast-levelling head. When you shoot with a tripod "readiness is all". After one-two minutes you are taking pictures somebody will probably arrive.
PS Or you must really look like a tourist. Nobody bothers tourists, also if they have a tripod. That's the mystery of "security" behaviour. Just wear a Japanese mask...