Can we try not to be sociopaths for two seconds? Just politely ask the person behind you if they mind. They'll most likely say yes even if they do mind, but they'll appreciate the gesture.

I personally find it impolite to lean back without acknowledging the person behind, given the tight seating arrangement (which everyone has common knowledge about).

The reclining feature of the seat is a vestige from a day when seats weren't crammed. Just because it exists doesn't mean you should feel entitled to use it without any consideration of others.