There is some incredible rubbish being spouted by some posters on this thread. I declare an interest, as I have a DS who is anaphylactic to certain types of tree nut.
If you have no personal experience of this type of allergy, the account of how easily a life-threatening reaction can be started, can seem implausible. Peanuts are a particular risk because they are more dusty than other nuts. As other posters have explained, some people do suffer a reaction from airborne particles, and the recycled air in a plane increases the risk of this.
A bigger risk is accidental ingestion of a tiny quantity of the allergen. Imagine you are allergic to peanuts and you visit a pub where peanuts are sold and eaten. Anyone who has handled peanuts (eg a bartender, someone who picks up your glass by mistake, someone opening a loo door after they have had their hand in a bag of peanuts)could leave a trace on your glass, or on a surface that you touch,. Once you ingest that, and it would be only a trace, you risk anaphylactic shock. This is not far-fetched - it has happened to us, and to someone we know.
Imagine how much increased this risk is on a plane where nuts are served. It's a tiny space in which a high proportion of the passengers would eat the nuts provided, then handle the armrests and backs of seats, loo door handles, taps, safety cards, you name it. And planes are often cursorily cleaned between trips, so there can be an accumulation of allergens.
This really is not the allergic being precious or attention seeking. And as for the cause and why these allergies are increasingly commonplace , not even the consultant who treats my son could give an explanation. In our case, it certainly wasn't caused by obsessive cleanliness - he was the most disgusting, mud-covered, dog-snogging child you could wish for.