Is it really such a hardship to refrain from munching on peanuts for a few hours if you have been advised they could be detremental to another person's health? Knock yourself out when you land; eat ten bags!
The problem with peanuts on board an aircraft is people with severe allergies can develop anaphylaxis by enhaling the tiny dust particles of a packet of KP being opened from ten rows away. With recirculated air carrying this about, is it really worth the risk to somebody?
The individual did not ask for a life threatening allergy. They have taken all the precautions they can by advising the airline and carrying medication, but as another contributor said, it only buys time. It is not a one jab cure all. Comparissons to people with allergies to man made fibres and such are really not relevant.