1) The labs don't want garbage books, they want interesting books that are rare and unique. They want high quality training data, random permutations of language style are fine, but what you want is unseen information, unseen patterns of thinking, unseen ideas.
2) Most pallet of books contains lots of valuable and interesting works, maybe 1-3% but sorting through them takes time, money, and energy, that's why the labs are starting to purchase by the pallet, it's because they already have a fully automated process so they can always beat any bookseller small or large on cost to find the books of interest and value in a pile.
3) Many of these pallets may sit for years before being sorted, and many of the books may sit for years before being sold, but these things actually do eventually happen, valuable books are found, and they eventually make their way to interested readers, this is the business model of used bookstores. Most books of value don't get destroyed or thrown away.
Destroying human art, knowledge, and culture is an essential part of the business plan for frontier labs, it is not enough to steal and regurgitate all the art and information in the world, you also want to make it inaccessible through any other means than the regurgitation machine. Don't expect the book burning to be an isolated incident, they are coming for every other form of stored human knowledge or art, and yes, unfortunately while scanning it they will have to destroy the original copy. And attacking the past is only the beginning.