Originally Posted by
Arcanum
That's exactly the point. By locking around a portion of the frame (be it the seat tube or seat stay or whatever) it forces the thief to cut the lock or ruin the bike. Locking just around a wheel means they can leave the (replaceable) wheel behind or, at worst, cut the (again, replaceable) wheel.
Again, a bike without a rear wheel is useless to the average bike thief. Locking security is mostly illusory: all a serious thief needs is to cut the lock or (often easier) the pole to which you're locked.