why is north korea available as your flag when north korea bans EVERYTHING


There are SOME people allowed access to the outside world. It's extremely rare — mostly Pyongyang's inner circle — but not nonexistent. The site doesn't expect there to actually be users from North Korea, but many sites have it as an option for database completeness reasons, and juuuuuust in case.
There's also the possibility of defectors or expatriates who aren't physically in NK but still consider it their homeland.
EDIT: Apparently there are approximately 12,048 accounts that list North Korea as their country, according to this thread. While I'm pretty sure most of those are a fake/meme listing, there's probably a few that aren't.