It is illegal. If this is something you’re worried about happening in your local area, I’d recommend making a fuss about it. If you can find evidence of specific people doing this, collect it and make sure the law is enforced. If you know a local library is being targeted, get them to take steps to minimize the risk of it happening. Things like making sure any large trash bins are readily visible to librarians or developing policies about just glancing in the trash every now and then to see if books have been thrown away might be helpful.

You can also probably help by keeping an eye out for books that may be targeted. If there are books you think might be subject to these sorts of actions, you could make a list of them and drop by to check on them every now and then. See if they’re on the shelves. If they’re not on the shelves, ask if they’ve been checked out. If they’re missing from the shelves and also haven’t been checked out, consider looking around to see if you can find them hidden away somewhere or buying a replacement to donate.

It probably isn’t simple to enforce this and to protect books from being removed from the shelves sneakily, because somebody has to put the work in. But if you care enough to do so, there’s no reason that person can’t be you.