People act differently when they are not accountable for what they say. You know that from reading every online forum ever. My facebook feed to positively civil compared to every anonymous forum.
Ideally you'd be able to use your real-world name, whether it's your legal name or not, but how can facebook tell the difference between a name you made up for facebook and one you use professionally or personally? Just how hard do you expect them to work to accommodate every keyboard warrior?
If you pick a realistic name and act like a real person, facebook will not be able to tell it's not your real name, and they have no interest in disallowing you. But you will have few friends, because you will not have a ready-made real-life network, so you will probably seek the company of other people who are using fake names, gathering around common interests. But some of them will use obviously made-up names, and they will fight because they aren't accountable, and someone will rat you all out.
This isn't facebook's fault, or their problem to solve. They don't want you turning their service into a duplicate of every other keyboard warrior antisocial forum on the internet. It's not in their best interest to be used as a tool for keyboard warriors and social misfits. Keyboard warriors and social misfits ruin the experience for "normal" people, and facebook is earning money serving "normal" people.
The lesson: If you've chosen a realistic name and want to be treated like a real person, then act like it, and watch who you keep company with. Don't piss off keyboard warriors and facebook won't know, doesn't need to know, and doesn't care that it's not your legal name.
Blogger or other well-known pseudonym? Create a facebook page rather than a personal profile. Be a facebook "like" or "follow" rather than a "friend".
Geeks want to discard any system that is not perfect for everyone, including edge cases and trouble makers. It's not realistic. If Facebook's rules don't work for you, then don't use it. They don't exist to provide a forum for you. They exist to make money, and they do so by providing social networking for real-life people, for people who want that.
Disclaimer: my slashdot name is not my real name, and I can't use it on facebook. I'm also a social misfit and generally pretty unsuccessful at social networking. That's not facebook's fault or problem.