the process of eating is not as straightforward as it seems. while manipulating food and chewing it, you do release tiny microscopic particles of it (especially true for something that has the consistency of peanuts) that end up floating in the air. somebody with high enough sensitivity will be affected, as one single strand of the protein responsible (or a single fragment) can trigger anaphylactic shock

edit: I stand corrected, somebody below has linked some studies done on this and it appears to be more about surface contamination, not air exposure. But then if it's surface contamination, that would be an issue from the previous passengers on the flight, not the current ones I think?