Jeez your right! I saw that pic one time before some years ago and I thought he had went around the seat tube....yikes! Most everyone I ever knew who had a U Lock always locked it around the seat tube, though a few didn't. But if you do as I said earlier and take the front wheel off and bring it along side the the rear and lock both wheels and the seat tube to a bike rack that is the best way. Or if you don't want to do that then buy a second U lock and lock it around the front wheel and the down tube then to the rack. But you shouldn't need two U locks if you just remove the front wheel. The one interesting thing about using a cable lock (in addition to the U lock) is that you could wrap it around the wheels and frame and your seat then to the rack, this would help prevent the seat from being taken.

I found this web site which details how to lock both wheels to the bike to the rack better then I can explain it: http://www.missinglink.org/Pages/bike_locking