I don't see it as a problem, but if it is a problem, it the school's decision which you have a problem with. I also don't see how they are abusing the system any more than getting a job elsewhere. It is also a money thing for these private schools. Having a proven winner coaching might bring in more tuition paying students.
Too many young coaches think they are ready to be a head coach. It used to be that coaches had to work their way up.