You're right, the parents do not have a right to DEMAND that others help them manage their children's food allergies or be provided with segregated eating areas. (By "abandon wheat eating" I'm sure you meant to say "abandon wheat eating AT SCHOOL")
But if they asked politely not to send peanut butter sandwiches to school (as they do at my daughter's school now,) I would graciously comply -- 1) because it's not a hardship, unless I choose to view it as a hardship, and 2) because I would feel some inkling of compassion for parents and children who have to manage such difficult health issues. (No, it's not my problem that these kids have some weird allergy to an otherwise harmless substance that most people can eat freely, yet it can kill these children within minutes... and for that I'll count my blessings.)
However, if I was the parent of a child with a life-threatening allergy, I certainly wouldn't rely on teachers or other parents to safeguard my child, even if special accommodations were allowable.
Also, I'm guessing that it's not the parents who are making the demands, but rather the school administrators who are trying to reduce their risk of liability.
Now I can't even read the full article because the link doesn't seem to be working, but roughly 1.5 million people suffer from a severe allergy to peanuts in the United States according the American Academy of Asthma and Immunology. That's a lot of people.