True, but rarely do they actually incorporate the donations into their collections. More typically, they combine the donations with books they've culled from their collections and sell them in "Friends of the Library" sales to raise money. Which is all good and fine, but not often what people think will happen.
Which are often immediately sold to the general public in a used book sale.
What's wrong with that? The library gets money and the book goes to someone that wants it. Win-win.
I have no problem with that. Selling used books as an individual is hard, since people will not pay a lot of money for them (since brand new books are cheap), so at least for me I find its not worth the time to sell them. At least then the public library benefits from the donation.