Slightly OT, but aren't we already discussing this in the big IAD-FCO thread?
http://flyertalk.com/forum/showthread.php?t=614668

Anyways, to answer your question. The Hawai'i 77s aren't configured for transatlantic flights. Sure the FCO flight will have more tourists, but I have this feeling that there may be a lucrative contract that UA just signed for some upfront seats. Or they're just wanting to get a big chunk of the tourist dollars. You can also note that perhaps on the IAD-FCO route, the average Y seat might lose money, but on the return flight, UA might be able to charge a slight premium for a nonstop flight. Also remember that the Euro is still quite stronger than the dollar, so that could be a reason as UA sees that the Italian traveler can put a lot into their coffers.