Everywhere I've lived, the public library has a massive annual book sale. Also, we keep building new branches -- at least 4 here in the past 20 years.
I don't doubt that they also throw many in the trash, but given that there's a steady outflow of books back into the community, and that total shelf space is generally increasing, I don't see why it'd be illogical to believe that no books got dumped.
My town hosts a sale twice a year. The materials are a combination of community donations and withdrawn from circulation items. Typically the sale lasts a week then on the Monday after the sale, teachers are allowed to claim whatever materials are worthy of classroom use. Everything else gets donated to a third party or trashed.
I don't doubt that they also throw many in the trash, but given that there's a steady outflow of books back into the community, and that total shelf space is generally increasing, I don't see why it'd be illogical to believe that no books got dumped.