Whilst most allergies these days are just trends and fads (bordering on the ridiculous sometimes), nut allergy suffered do get my sympathy as they can fall ill due to particles in the air. A cabin air moves around the cabin and air is moved by people moving throughout the cabin then it can travel several rows.
For the safety of the sufferer and the airline's bottom line, I'm more than happy to prohibit the sale of any items that may result in a serious medical and or diversion.