would be much harder to expect people to go without cheese for instance, or dairy based spreads, or to request that babies not be fed milk on a plane, and yet the life of the dairy allergic person is surely just as valid as that of the nut-allergic one? And if not, why not
Because people are not always clear as to what they have. Allergy and intolerance of a food product are two different things and often people have an intolerance but claim they have an allergy.
The difference is an allergy can kill you, an intolerance causes a lot of painful and uncomfortable effects.
If eating or drinking a food causes a visible reaction, it's an allergy that can lead to anaphylaxis which can kill. If you eat something and it causes uncomfortable side effects (wind, loose stools, pain in tummy etc) it's an intolerance and won't kill you.
Often dairy and wheat are intolerances and won't kill and are not usually air born.
My friends peanut allergy means she can't have almonds, peanuts or chickpeas and she advises airline before flying. It's amazing how disgustingly selfish people are when everything containing those products is removed from the plane. She had a reaction and the plane had to emergency land because someone ate a Bakewell tart they had on the plane and were told not to eat.