On most airlines, the seat is made to recline. If you don't like this, book a seat with more leg room, or book on a plane which doesn't have reclining seat.
I do mind when they just smash it back as hard as possible the instant the seatbelt sign switches off. I also very much mind when they do it while I have stuff on the table in front of me, especially since the seats seem perfectly designed to smash cups and water bottles. Or even worse -- laptop screens.
It's fine that people want to recline their seats, but they should be just a tiny bit mindful of the person behind them.
