By law in Italy, you have to produce ID to buy tickets for a football match. I understand it is to stop hooligans. Another thing to bear in mind is that (again by law) Italians carry their ID cards around with them at all times. So for them, it probably isn't an issue. For unwitting tourists, it is something to bear in mind.
Still, we had a nice wander about the old part of Bergamo (Bergamo alta). But it was heaving with well-dressed Italians. I have never seen it so busy. It was like the streets were peopled with ants. Probably because it was a Sunday. It was sunny. And there was a home match.