Schools have a limited budget and many choose books that will help the most students - help them read more, are of interest to more of them, cover a subject no other book in the library does, expands a subject many students have shown an interest in by checking out all the books on that subject.
Some schools buy the Scholatic books because that’s what they have a budget for - hold the book sales and they get a percentage of the sales to fill their shelves. What is in the sales brochures, both the ones they distribute in the classroom and the big book fairs, is pretty political too. My aunt writes Young Adult books, mostly biographies, and she really only makes money when her book is in the brochure (sells many times as many when her books are in the brochure as when they are just offered to bookstores or to school districts for libraries or classrooms). There is nothing controversial about her books about presidents or first ladies, but she’s competing with a lot of other books. If the book is offered for free to the librarian, that librarian may say sure, we’ll take that book.
There are a lot of baby board books, but does a school library need to buy every one? Is Babies Everywhere that much better than the 10 other baby board books the library already has? Does the hs library need that book? Does the fact that it appeared on the book list a certain group wants banned mean that the school district is really banning it or could they have come to the decision independently that they just don’t need that book, or that it is outdated or that they are removing it because no one ever checked it out so they are making room for a more modern book that more kids will check out? Huck Finn was banned, but if a library had it today, would a lot of kids check it out?
I think if kids live in an unincorporated area that has no public library, and the state doesn’t provide access to a public library, the state should change that. In Colorado anyone can get a library card and borrow from any library in the state, and return the book to any library in the state (or borrow an ebook). It may take some double registering (get a card at your home library and then register at other libraries) or using interlibrary loan, but it can be done. When we lived in Florida, there was no public library in our area (not really a town) but my kids had access to all the reading material they needed, either through buying it at a bookstore, Target, school library, online, borrowing from friends, ordering online.
I agree that kids are more likely to read something if they have easy access, but I think in the 21st century they have pretty easy access to everything.