my wife is a teacher and not only gets no pay for days off but has to pay her sub

She has decided to just substitute teach instead of taking a permanent job for a few reasons, first off she don't have to pay to update her certification and makes more money as a sub without it. Secondly and this is the one that pisses me off, if she has an emergency and has to call in she is required to pay for a sub which makes more in a day than she does even if no one takes the job and she is just paying the school. How the fuck is this even legal?