I can live with the person behind me thinking I’m an asshole.
The airline offers the facility and I won’t sacrifice my own needs for fear of upsetting a stranger.
I suspect they’re not the only person around you who thinks you’re an asshole.
But when you find yourself in an uncomfortable group situation, it’s good to ask how you can make it better for everyone, or at least not worsen it. “I paid so I’m doing whatever it takes to not inconvenience myself in the slightest” is the origin of “this is why we can’t have nice things”.
It’s different when there’s a compelling physical need here. If the person in front of me has a hurt back and can’t bear sitting upright, and I knew about it, I’d put up with it as best I can in the interest of we’ve all got to get along. But in the scheme of things, not that many people are unable to sit up and not crush their neighbors.
How would you know whether or not the person in front of you has a compelling physical need? Are they supposed to explain their health concerns to you in the hope that you deem it sufficiently acceptable for them to do what they can to be comfortable? It’s unreasonable to expect someone in physical pain to suffer the indignity of explaining themselves to someone they don’t know.
It may well be an inconvenience to you if someone chooses to recline their seat but nobody owes you an explanation. Expecting them to justify themselves to you isn’t a reasonable stance. And given that you don’t know what someone is going through, whether they have a hidden disability, whether they’re in chronic pain, I really wouldn’t advise asking, let alone shouting at them.