I dunno but it works here. Why take the chance on sending a PB sandwich with a small child who might get into his lunch box and share his lunch with his buddy who might have an peanut allergy. I don't have any kids with allerges but I'm very thankful for teacher who take the timie to check.
We're high school. We don't eat lunch with our students. Every other week we're on hall duty keeping track of things that don't occur in elementary schools: For example, students sneaking out to the parking lot to smoke or leave campus, students sneaking out the back doors to have sex. Our lunch is 26 minutes. Our real lunch concern is, "What's in your water bottle?" If we add checking the contents of student lunches, the teachers aren't going to get to eat at all.
I don't know why people keep arguing it "isn't enforceable" when lots of schools, districts and classrooms do ban it and do enforce it.
Well, let me give you a couple examples of why a ban isn't really possible:
Our students arrive (50% on busses, 50% in cars), and on any given day at least a hundred of them have stopped by a fast food restaurant to buy a breakfast biscuit. They're allowed to eat these as they walk in from the parking lot, in the open-air front courtyard or in the lobby area, but they must throw all food away when the bell rings and the classroom hallway doors open. Around here Chick-fillet is the favorite fast food restaurant. Unless you suggest that teachers meet students at their cars to check their breakfasts, it's impossible to police what students have before they arrive.
In that same thought process, students who eat peanut butter (or maybe Nuttella -- that contains nuts too) for breakfast may well come in with some peanut residue on their hands or on their breath. We can never police what people do before they arrive.
At lunch, we have 1600 students moving through the cafeteria. I'd estimate 40% of them bring their own lunch. That's perhaps 640 lunches to check.
Eating in class (or in the hallways between classes) is a problem in high school. Between kids who don't eat breakfast (we do start school at 7:00) and kids who are forced to eat lunch early (my assigned time is 10:30) and are then hungry by 6th period, kids do sneak foods in. Sometimes we catch these, sometimes we don't. But no one's checking food that's hidden in backpacks and isn't really supposed to be there at all.
Many students stay after school for sports or theater practice, club meetings, tutoring and other activities. Most of these students bring a snack. What's easy to keep in your backpack all day long without refrigeration? Peanut butter sandwiches or peanut butter crackers are near the top of the list. Kids tend to eat these in the hallways or the locker rooms as they move into their after-school activities -- not in the supervision of teachers.
These things may not be true at the elementary level, but they are very true when you're teaching teenagers. Teens have more freedom and more access to food, and if we claimed we could police the food that comes in, we'd be lying. If we claim our school is peanut-free, we'd be setting kids up to believe they were in a totally "safe" zone, which is impossible to enforce.
Also, keep in mind the number of peanut-allergic students in our school during the 2011-2012 school year: Zero. Why would we encumber 1600 students with a rule that would benefit no one?
Taking the item away from the child, note to the parents regarding the food policy. Having to live their life knowing they sent something that caused harm or death to another child

It can be that serious for another child. I have seen so many parents act like lunatics over their "right" to send peanut butter for their kid and I seriously can't understand it. If you know you are putting another child at risk and do it anyway you are just an idiot and a cruel selfish one at that.
I think you might be the exception rather than the rule regarding peanut/food allergies. My ODS is entering 3rd grade and there have been at least 1 or more kids in each of his grade levels with serious food allergies and I know at least one serious peanut allergy preceded him into Kinder as the teacher mentioned it and one room was deemed totally nut free. My DH teaches middle school and has for about 12 years and he knows he has had several food allergies/peanut allergies over the years. My Mom spent almost 30 years in education and had plenty of allergies..some of them life threatening/extremely sensitive to oils/residue as well.
My YDS tiny preschool (as in less than 50 kids in the school) had one child with a life threatening allergy..he would react from being in the room with it/oils on the hands/objects. As in call 911 and use the epi pen reaction. The school was 100% peanut/nut free.
Plenty of schools or individual classrooms have a peanut/nut free policy and enforce it. If something is questionable (and really..aside from peanut products or products known to contain peanuts what is questionable about it? Nobody is going to snatch their ham and cheese and think it might be peanut butter..) then they might offer an alternative item and toss the questionable item. Really not that hard and lots of schools have done it.
Perhaps it's worse in some areas than others, but I never, ever hear about this problem in real life. I know one kid who has a mild peanut allergy, and he's responsible about eating only things his mom prepares.
That's funny..my DH is diabetic, has worked extensively with a nutritionist and was never told to keep it on hand nor was it ever deemed critical to his survival as a diabetic. I can't think if the last time he even ate it.
My husband is diabetic as well, and peanut butter is one of his go-to items. He loves it, we always have it at home, and it's an easy thing to keep in his desk drawer at work -- it's cheap, and it stays good a fairly long time without refrigeration, making it a good "hey, my blood sugar is crashing" choice. If your husband isn't using peanut butter, it's probably because when he met with his personal nutritionist to discuss what'd work well for him, peanut butter didn't come up, so it wasn't included in
his personal diet plan. Doesn't mean it may not be an integral part of someone else's diabetic eating plan.
Since this is a school-related question, I'd say that on average I have a diabetic student every other year. In my personal experience, diabetes is much more common than peanut allergies.