I'm sure every school will have different reasons for their acceptance criteria, just the same as with universities etc. Some will have built up a reputation that they wish to uphold and want their cadets to reflect well upon the school. I would also imagine any school that trains cadets as part of a group would want to ensure they can keep up the pace. Some may also feel it is not right to train students that don't have a good chance of success.
On the flip side, some schools may be far more flexible in training students who will likely need a bit of extra help and without meaning to sound cynical, there will always be some that will happily take your money regardless of whether they believe students will succeed or not.