I don’t entirely agree this is the case. To some sure, but to many others the disrespect is not in the humiliation, or punitive act against the hero, it is in the fact that they feel it has not been properly motivated or set up. To them it’s like the story telling them, Luke’s different now, deal with it, and if you can’t, that’s your problem.
While harsher than I’d phrase it… that’s basically it. Either you can make that leap, or you can’t.
There’s nothing wrong with you as a viewer or a consumer of entertainment if you can’t, it’s not a personal failing or anything like that. But Luke is different now. He HAS to be different, and maybe he’s now different in a way you don’t like CONCEPTUALLY. And at that point, if you’re already firmly disagreeing with the very notion that he HAS to be different, and even more strongly disagreeing with HOW he’s become different - it’s going to be very hard for any story to keep hold of you, because you’re going to need convincing this idea is justified, and you’re automatically disinclined to buy it. It’s a relationship between movie and viewer that is instantly confrontational. The movie wants you to accept this is how he’s different, and this is why he’s different, and there’s only so much time (even with two and a half hours) to get into how that happened. And you want the movie to convince you that the premise isn’t mean-spirited, or stupid, or capriciously punitive. You want reassurance from the filmmakers that they’re not just doing it to do it, to be disrespectful to this character as a means to make these other characters (that you don’t really like much anyway) stronger. It’s seen as a transaction, not a story. “Oh, so you make my guy (note the possessive) into a big loser so these other people get to be winners at his expense. No thanks.”
There are limits for many viewers as to what “Luke Skywalker” can be, and should represent, and those limits are broken from jump the second The Force Awakens says Luke has disappeared for ten years. Because he’s not around in that movie it’s easy to not wrestle with the idea too much, but The Last Jedi has to dig into why those limits have been transgressed. Viewers willing to accept the idea that “Luke Skywalker” is a vessel for ideas and concepts that don’t line up precisely with what we’d seen in prior movies seem to be enjoying The Last Jedi more than those who essentially reject the notion out of hand.
“Luke wouldn’t do that” is a very strong statement, and definitely a valid one. I think he would, because I think Luke is capable of doing a lot of things depending on what ideas need to be expressed by the writers and creators in charge of the stories he’s in. I get why people would balk at his whole situation in this movie. But I don’t balk at it, and I in fact love what happened because it made him a richer, more interesting, more empathetic, and more lovable character. His failures didn’t ruin him. They eventually made him even greater. That’s inspiring to me. Even good people fall down. That doesn’t make them bad people. It just means they need to work harder at remembering who they can be, and moving back towards that light.