I heard this, and the story goes that the 340's are very expensive to maintain and fly. They're floating around available for that reason. Whoever holds the leases wouldn't want a short term lease, leaving US with an expensive airplane long after they cease needing it. Hence, a weight restricted A330.
Now, hear me out a second. This wouldn't actually be such a bad idea if they would think outside their nightmare accountant's brains for a bit. Since the silly thing will be weight restricted, why not reconfigure two of them to an industry leading pitch and service? Take out heavy seats, give generous leg room on a very long flight, and formulate an industry leading service. Asian flying is very competitive, Singapore airlines is in they fray! They have to run it light anyway, why not tout it as their service? They could actually make a good (gasp) name for it, then whenever the 350's come they could do the numbers and see whether they want to return to domino seating or not. The thing is, assumably there will be an Asian population to fly as well, right? Well, guess what? Culturally, that population does not complain. They simply do not fly you again. And they tell their friends. Wouldn't it be nice if US actually planned something out for a positive outcome?
Okay, I'll wake up now.