It's mandatory as in over that you go into overtime and contracts must have a maximum yearly overtime limit, which in cases like this is often ignored.
The problem with Prato, as written in the article, is that it is plagued by illegal practices, so even if your category contract is supposed to protect it, you may simply not have a contract or they can just ignore it. Contractual disputes are solved in front of a judge, and it takes a long time to get a hearing.
It's simply a criminal environment, which is partly tolerated partly fought by the authorities.