Wow, how rude! Were they Italians or foreign?
I too noticed that in some places (but not Italy) people tend to help themselves like that. In some places, they even tell you to your face that they don't have change, but they'll give you a candy or a pack of gum, in the hope that maybe you'll say 'no thanks, keep the change'.
One funny story happened to me right here in Italy, related to tipping, just a week ago.
I went to an agency to help a friend send a package to his home country, and the sum he had to pay was not a round sum, but he had to receive change.
The sum was either 21 or 16 or.. something like that, the idea was that my friend gave the agency guy a paper bill and 1 euro, so that would get rid of the change and receive a paper bill in return.
Anyway, the agency guy was expecting a tip, so here's how he prepared the change for my friend: he was supposed to give my friend 5 euros in return, so he prepared 4 euros using four 1 euro coins plus 1 euro using many, many small coins, in the hope that my friend would leave the many coins to the guy. How rude was that ?!