Are you okay putting Penthouse magazine (or similar) in elementary schools?
I'm guessing you aren't... people have to draw lines somewhere on this. There's a difference between restricting by age, making available to all and actually assigning material to children. Not all materials are appropriate for elementary school libraries. And I'm not even talking about The Color Purple specifically.
Also, none of this stops a parent from buying a book for their children they feel is appropriate for their child.