I think it’s simple demographics. We are b”H growing, so we need more and more teachers, so perforce, we need some inexperienced ones.
If, as you say, those who won’t commit to stay the whole year no matter what shouldn’t teach, they will obviously begin other careers, and that will deplete the supply of teachers. I don’t think forcing them to commit (contractually or morally) benefits klal Yisroel.