The first and most obvious objection is, why not give the books to the poor? They need stuff to read. Or to prisoners? Or to sick kids? Or to struggling independent booksellers? It doesn’t cost a thing to give something away, right?
The problem is the situation for a library is more complicated than when you just take a bunch of old clothes and unwanted porn down to the Salvation Army. A library book is stamped and bugged and cataloged so that the library knows that it belongs to them. When a book is given away or sold, the library has to go through and remove all that crap, so whoever winds up with it can prove they didn’t just steal it off the shelf. I’m not kidding about that, either — some people who wind up with such books helpfully return them to the library.
And we’re talking about a lot of books here — these libraries are having to cut down their stock in a hurry. Imagine you’re the manager of a library, and some accountant tells you that you need to get rid of 100,000 books, and do it in a week. You really have two options. One, you can get a bunch of academics to scour your collection and painstakingly rate each book according to its value and importance. Then you can hire a bunch of people to take down the 100,000 least important books and painstakingly stamp and debug them, one by one. Your second option is to get the computer to spit out a list of the 100,000 least borrowed books, and hire a few people to walk down the aisles with their arms out, throwing those books in a shredding machine.
That second option is much quicker and much cheaper. Sometimes you can find a paper recycling centre that will pay you for the pulp, so destroying the books leads to a net profit. Nobody likes it, but for a librarian it’s like your best friend just got bitten by a zombie and you’re the only one with a gun.
Also, remember that the stuff worth saving is buried among a lot of other books that are basically garbage. Though everyone realizes that extremely valuable books are going to inevitably get caught in the same net, there’s not much that can be done about it. Nobody is going to order a first-edition Moby-Dick from a library warehouse if the 2011 reprint is sitting right there on the shelf. A computer list that ranks books by popularity can’t tell the difference.
Another downside to this option is that you have to ensure total destruction. You can’t just throw the books in a Dumpster for some asshole to come along and grab later. If you go the Dumpster option, you have to tear out chapters so that people won’t want them, or just fill the Dumpster with detergent. You don’t want people to get in the habit of treating your Dumpster like the clearance rack — it’s dangerous and messy for everyone involved.
There is much more at the link.