Do you literally think that elementary school libraries should stock any book ever published, regardless of content? Nobody actually believes that. There will always be a line somewhere, the disagreement is just over where the line should be drawn. That's why all the talk of "book banning" is disingenuous.
They don't have to stock everything. But it's not about stocking regardless of content - it's about refusing to stock specific content. They're not removing the least interesting N% - it's very targeted.