Open source developers working at the one, heavily monitored, most likely site-restricted cybercafé in Pyongyang?

Syria, Iran and Cuba are different matters entirely, but people in North Korea are limited by having no Internet connectivity in the first place, than by being blocked by sites in the US. You may as well ask why the civilizations of Andromeda are stopping us from selling our wares there.

So if they're already blocked off, why is it a legal requirement for US companies to block them?

I can't speak as to the policies of the US government, but aren't blocks like these related to the more typical physical sanctions of delivering products to a country? With the Internet, you can just VPN around these blocks anyway, so anyone in NK who wanted to access this stuff and had a connection could do so.

That said, since the only people in North Korea who are likely to access the Internet are the people in charge (who we "don't like"), it doesn't seem unreasonable to block their access to software they might use even if it's easy to circumvent anyway.