In other words: there are plenty of reasons to not stock a book that are not partisan or doctrinal. We don't expect public schools to pay for expensive medieval manuscripts, for example, or to stock books in languages that aren't represented in their district.
I don't see how pornographic bans wouldn't qualify as "doctrinal" though they are not particularly partisan.
You're leading with the assumption that you and I (or anyone else, really) agrees on what "pornography" is, much less that we agree in a non-partisan context.
The context here is that there's been a significant effort in the last ~18 months to reclassify LGBTQ fiction and non-fiction as pornographic and have it removed from school libraries on that ground. Justifications for that vary, from the more staid pearl-clutching ones, to rehashes of old and dangerous stereotypes about gays predating on children. That is absolutely a doctrinal concern, even if the nominal topic ("don't show children porn") is one that appears reasonable and uncontroversial on face value.