In England you can use whatever name you like, as long as you don't have the intent to deceive. (This is tricky with doctors and banks). So, legally, I could be "Bob Smith" and "Ann Jones" at the same time. Why does Facebook get to tell me that I only have one name when my government is fine with me having multiple names?
I've never tried to use a false name, but it probably would be easy for many things (utilities, local government) but difficult for others (bank account).
Because Facebook isn't your government. It is a non-government entity with a very specific vision. If it finds that the use of fake names hinders the experience of other users and there by Facebook Corp., it should be allowed to require real names.
Other than for a small number of reasons due to protected groups, businesses generally have the right to refuse business to anyone for any reason.
Do I think Facebook should be allowed to block black people? No.
Do I think Facebook should be allowed to block pseudonyms? Yes.