Gay Penguins in the School Library

The case of the gay penguin book in the school library intrigues me:

A picture book about two male penguins raising a baby penguin is getting a chilly reception among some parents who worry about the book‘s availability to children — and the reluctance of school administrators to restrict access to it.

Complaining about the book‘s homosexual undertones, some parents of Shiloh Elementary School students believe the book — available to be checked out of the school‘s library in this 11,000-resident town 20 miles east of St. Louis — tackles topics their children aren‘t ready to handle.

For now, "And Tango Makes Three" will stay put, said school district Superintendent Jennifer Filyaw, though a panel she appointed suggested the book be moved and require parental permission to be checked out. The district‘s attorney said moving it might be construed as censorship.

Here's the part I don't get. Presumably, the school was free to buy or not buy the book when it initially did so. Not buying the book would not be considered censorship, I don't think. So why is deciding to remove a book (or putting it in a separate section) considered censorship? It's as if school libraries have one chance to filter out what they deem to be unacceptable content. Once they buy it, they're stuck with it. Seems odd to me.