Some are franchised out to different airlines who may not have the same policy or not be advised of special arrangements. When many passengers open packets of peanuts the allergenic properties are released into the air and can enter the eyes nose and mouth, causing problems in mid flight for some people with allergies. Seek to educate. One in 200 people has peanut allergy. If you consider that the allergic person's family and friends will be flying with them, that amounts to a lot of passengers.
Airlines have no control over passengers bring their own packets of nuts on board.
This would probably not be a problem. If you are concerned, discuss your proposed flight with your GP or specialist. Some airlines may request written medical evidence of the allergy. Take some high energy snacks with you in case flights are delayed. If you are successful in securing special arrangements ensure these are in place for all connecting and return flights.