I don't know about other schools, but for USC, they tell you that when the admission cycle is over. For example, you didn't get accepted (i.e. were not pulled off from the waitlist) until August and got a letter saying that "sorry... you are not offered etc..." Then, you can ask why and they tell you WHY.
Before that, USC doesn't reveal anything.
I think it's similar for all other schools too. anyone with other opinions?