Here is why this has become such a problem:
All the way from 2nd to 8th grade, my son was taught in his English classes that a story's THEME is THE LESSON IT'S TEACHING YOU. Every fucking year! I would literally ask him "so how are they defining theme this year" and he'd be like "they still think it's a moral" and then he'd be treated to Yet Another Rant on this topic.
It wasn't until last year - 9th grade, not until freaking high school - that they finally stopped this bullshit. But I promise you, for many kids the damage has already been done and they probably don't even notice that the teacher is now calling things like "friendship" and "growing up" themes instead of things like "be nice to your friends" and "it's important to learn new things as you grow up."
I know this bc I have my students, who are mostly college freshmen or sophomores, make a lesson about any topic they want, and a couple years ago a girl did one on writing a story and SHE defined theme as the lesson the story is teaching.
My background is in science ed. I have NO IDEA where in literacy ed people decided this was the way to go, but it seems to be endemic now. And I think it's not just factually incorrect but downright fucking dangerous.