More and more, people seem unable to deal with disagreement or conflict. Some readers don't get that it's impossible for any human being to write an entirely "neutral" book that doesn't reflect author voice or opinion in some shape or form, even if subtle. Interestingly, those readers often only seem to complain when the author's opinion or voice suggests a belief in something that the reader feels strongly opposed to. Then it becomes "the author pushing their agenda". If you're going to dislike a book for an opinion or issue you disagree with and that an author seems to be framing positively, to me the more logical response is just to say that plainly, instead of acting like an author should hide their opinion/belief.

There are some books where it can feel like an author is beating you over the head with an idea, but to me that has more to do with the writing than the expression of that idea.