First, I'm not sure that I am in love with your phrase "posthumous censorship." Imagine if changing sensibilities or racist or sexist language were to change the rating of a movie from G to PG or PG to PG-13. Given that circumstance the studio might quietly edit the original to preserve the original rating.

It is the same thing with books, when the publishers announced that they would no longer publish several of the lesser-known books from Dr. Seuss' back catalog it was to preserve the essence of his work for young children, while not exposing them to more mature or offensive content. See Dr. Seuss goes to war for examples of how racist Dr. Seuss could get.

I don't think most parents are too lazy to be good parents and want to outsource their jobs to large media conglomerates. There are times for having thoughtful conversations with children about what content is acceptable in books, movies, and video games. There are also times when a precocious young reader is devouring their way through young adult series at a rate where the parents could never keep up.

We rely on libraries, schools, bookstores, publishers, and reviews to help us make good choices with our kids. I have certainly found children's books from my grandparents' collection that I would not want young kids to be reading. Little Black Sambo maybe an interesting primary source that helps us understand the ideas about race the people were imbibing seventy odd years ago. But I don't want young children picking it up and internalizing the racial attitudes of the book.