It seems the problem is that before 1985 lead was used in the illustration ink of most children's books. (BTW, children's books here are defined as books for those 12 and under). Now, the government is very careful to say that these books are not banned. The government isn't telling anyone to throw out these books. They just have to get them (or a large enough sample) tested to make sure that the books don't have excessive amounts of lead.
The problem is that the government is asking for expensive tests. Most used booksellers don't make the kind of money that would allow for those expensive tests, so guess where the books go?
Summary of how things stand (to the best of my knowledge):
- "Vintage" or collectible books are safe. The government assumes only adults will be reading them.
- Library books are safe for a year so that the government can get the required testing from said libraries.
- Used booksellers either have to get the expensive tests done or chuck the books. It seems the most sane financial decision is to just chuck the books.