In a way it is. They have to cut material that the ratings board disapproves of. NC-17 and unrated films often get little or no exposure since most theaters won't show them and movie studios won't back them.
Fair enough. Though as I said, specialty collections
already filter material in libraries based on what is age-appropriate. It really wouldn't be adding anything new. And I wouldn't call tailoring collections to be appropriate for a particular age group "censorship." It's being a responsible educator, just like a responsible parent won't let their six year old discover hardcore porn sites or watch a movie like Kill Bill.
Having served on a board that dealt with challenges to public school library collections in the past, I saw how the process for developing collections works. It's very rare that a challenge results in removal of a book from the collection (it didn't happen once while I served). The people who serve on these boards have no interest in "censoring" material, they want to create age-appropriate collections. Beyond that, it's the student's and parent's decision about what books they read. More informative labeling could help facilitate that process.
In short, I think wailing about it being "censorship" is conspiracy theorist paranoia talking, no offense. Gods forbid we be reasonably sensitive to people's proclivities and develop age-appropriate library collections. I wouldn't want this taken too far in the PC direction either, but informative labeling can be useful. In fact, I don't quite understand why other forms of media already have systems in place but books don't. I might poke my friendly neighborhood librarians for more information about this.