You are wondering why I am annoyed that they get my seat? Because it is my seat, I paid for it and it does not matter if the trip is an hour (although less important) or ten hours. I know I can get bumped out at any time - I can live with that but that it is most likely an op-up who got my seat, that annoys me.

I could not care less if staff rides up front and I am in the back. It is their perk and airline staff is not underpaid nor am I. Different perks comes with different companies.

I am pissed off because of
a) the blatant favouritism played by the staff (and as you say, management probably knows)
b) that someone who was not in my seat one hour prior magically appears there and is served an M-meal.

(I am unsure of your last remark and if I came out completely wrong, then my apologies)