One would think that a civilized society would, in time, come to some accepted standards for civilized behavior. We are not talking here about anything so crude as laws; we are talking about simple decency towards your fellow humans. We do not require laws to tell people to give up their seat for a pregnant woman, or help an elderly person across the street, or refrain from cursing at waiters. We expect people to do that out of common decency. Those who would refuse to follow such simple rules simply because they are not laws are known as selfish, antisocial monsters who will likely die friendless and alone.
An airplane is an enclosed environment. For most passengers, there is not very much extra room. Space is tight. No one can go anywhere. We are collectively trapped. It is simple decency not to crudely and selfishly recline your seat into the face of your unfortunate neighbor to the rear. Yes, you could recline your seat, legally; but that would make you a selfish, antisocial monster. I trust that you are not that. I trust that you are a good person.
The idea that if you want people to be polite and considerate you should pay should pay them for the privelege should be a reductio ad absurdum of consevervtarian logic, only they’re willing to make the argument themselves.