“All ponies must be equal, dont you see? Books create conflict, draw flaws and differences out into plan view and if we want peace we need true equality”
 
Perhaps the princess was selfish. Stupid and maybe even inherently evil. She hid away these leather tomes into her private home for her own enjoyment. The Queen found out and gave her a choice out of the generosity of her heart. Either renounce her perverted love of books and help destroy her own collection or burn down with it.