The fact that a company isn't more powerful than a nation and that if you want to have German users you must follow Germany's laws?

They are not deciding what Facebook must do in general, but what Facebook must do so that they can sell Germans to their advertisers. Clarification (edit): With this I mean that they don't have to make pseudonyms available to, say, the French.

Don't know why you're getting downvoted. I absolutely agree that it is not the place of regulators to decide which features a website must or must not have. Let the German people decide for themselves what kind of usernames they would prefer.

Well, Facebook have an office in Germany for one thing.

Here's some job postings: https://www.facebook.com/careers/locations/hamburg and their Facebook page for the office https://www.facebook.com/FacebookHamburg

Because they want to do business in Germany? Or do you assume that corporation rights are more important than individual rights?

Don't worry though. The communists in the UN will soon decide that Facebook needs oversight.

Germans have historically thought they can dictate their wishes on others.