It ought to have been stamped and Italy is legally required to stamp US passports according to the Schengen Agreement.
According to the Schengen Manual for Boarder control Officers (translated from a German version I have seen)...
"Passports of third country nationals must generally be stamped when entering and leaving the Schengen Zone. Date and point of entry/exit can be ascertained by the stamp and the adherence to the authorized number of days of stay (90 days in 6 month) can be verified.
Border Police must therefore stamp the passport of third-country nationals even if controls have been relaxed due to exceptional circumstances.
The following documents may not be stamped:
-passports of citizens of the European Union, Norway, Iceland, Liechtenstein, and Switzerland
-passport of diplomats.....
Stamps should be, if possible, entered in chronological sequence, the exit should be documented by a stamp nearby the entry stamp to simplify the calculation of allowed stay. Stamps should be levelled horizontally."
This also explains why, my EU (UK) passport is never stamped. Even if you ask for a stamp they are not allowed to give you one. I guess that not having to stamp everyone's passport means some Boarder Guards forget to stamp those who they are supposed to stamp.