So the PCs can't die unless they want to, but it's perfectly fine for any NPC to die without consent, and you're blaming the players for not treating the NPCs like people? Do you not see the problem with that?
I've never seen it work that way. In my experience, players who think of NPCs as expendable cannon fodder are just as likely in lethal and non-lethal games. Some players just aren't that interested in the in-game narrative. I've even had players like that in my non-lethal games. It's not a problem as long as they don't piss all over the story for everyone else. With that said, though, I've definitely had skeptical players join one of our non-lethal campaigns and then fully buy-in despite their skepticism. Losing (or failing) a beloved NPC hurts, sometimes more than losing your own character.
As I suggested earlier, most consumers of fiction don't seem to have trouble engaging emotionally with narratives even when they know that the protagonists are likely to survive. Indeed, most bestselling fiction (whether film or novels) is far more predictable than the epic-but-not-especially-lethal D&D campaigns I've run. Sure the PCs are likely to survive, but the plot is dynamic because each session is subject to the actions of multiple personalities and the ever fickle dice. A big part of the fun for the players is trying to figure out a way to achieve their goals despite the ever changing plot knots they create.