Just to add on to the “keeps the doctor away” idea:
Not only does a sick teacher cause a lack of learning, they also increase the cost of health care for teachers which is a tax burden on everyone. It’s a way of preserving the already strapped school budgets, saving money for the taxpayers, as well as keeping teachers healthy (and the other kids, since they all interact with the teacher). An apple a day could be key to public health policy! Perhaps a per diem apple is what will pass for affordable, nationalized health insurance in the future. ;)