Maybe I'm missing something here, but my understanding is that those residency slots are funded by Medicare, not the residency program or the hospital - so it's not like it's a financial loss for them.
Residents can get sick, have to take maternity leaves, you might get short staffed at a shift etc...I would always want to have more residents around then less, especially when that isn't coming out of my own programs pocket. If you don't fill that spot, you may not get the funding for it anyway, or have the funding pulled permanently. That's happened to programs that routinely don't fill their spots.
Now, rusted fox's point is a valid one though. I can totally see pd's doing that. Where I trained, the Saudi government was funding spots that were reserved for its own residents.