It's just a cost/benefit analysis on the part of parents, administrators, board members, or whoever. Sure peanuts and peanut oil are in lots of foods, but if you can reduce their prevalence by 97% simply by asking parents not to send their kids to school with PB&J sandwiches, it might make sense to do so. As for the kids not learning to take care of themselves, I can speak from experience and say that those kids face more than ample opportunity: birthday parties, restaurant outings, well-meaning mothers of friends, ... the list goes on.

To restate, it all comes down to somebody deciding that reducing a 1 in 10 chance of something *really* bad happening to 1 in 500 is worth asking the rest of the kids to forego PB&J sandwiches at school. If it's that easy, that's all there is to the decision. It makes good fodder for people to huff about on talk radio, but most of those directly involved probably don't mind it so much.