His reason, although clear, is also plainly wrong. Such usernames although bizarre may be encountered by SystemD so it shouldn't break when it sees them. Computer programs, particularly important ones, should be conservative in what they emit and liberal with what they accept and that means not breaking when they encounter weird but technically permissible usernames. His response should have been "Golly, that's a weird username, I didn't think that was possible" and then fix the bug.