This still burns me up. I only signed up for that reason, and it seemed like a great idea, limiting it to people with institutional .edu emails so it was like a modern interactive yearbook. Then overnight it was something else. Like that old commercial for roach traps where they think they're settling down in a nice comfortable living room and then suddenly it's revealed they're trapped and are being poisoned.
I think some people legitimately do. If someone is pretty close to the typical mainstream archetype for their culture, it's easy to get caught up in idealism about how great things would be if all books were open.
The more one sees outside of that bubble, the more apparent the problems become, but being in the bubble reinforces the positive aspects of that model.
People just got tricked in to doing this because Facebook was centered around connecting people at college, where you interact with people in real life. It was obsolete and an aberration the moment Facebook decided to open up to the wider world.
Rate limiting solves the problem of bots crapflooding the service and turning it into a sewer. Not every account you find objectionable is a bot.