well the thing is that NCSOFT do have COX IP and trademark registered in the US, and thus violation of those copyrights and trademarks do fall under the US law.
They may have a set for the Korean area too, but that wouldn't apply here, but in short, they do have that stuff registered in the US with the US copyright office.
So more than likely if crap hit the fan and they do make a move, it would be played out in US court uder US copyright laws.
And NCSOFT do have offices in the US.
But also don't forget about the WIPO Copyright Treaty (which have 186 nations as members) and the Berne Convention which the US, and most of the world ratified and signed and agreed to except a few African nations in the south east portion and many countries in the Middle East. Originally the US refused to sign and become a party to the Berne convention in 1988 and the DMCA also covers violations to those treaties. Thus violating the copyright of a company in Korea still could be upheld and the person could be held liable under US law.
Now how it works in say Germany, may be a bit different but since the WIPO Copyright Treaty is basically an international law, it probably applies there too and vice versa if someone in the US took copyrighted materials of German origin and the German company got wind of it and put in a complaint.
From the copyright website addresses in the FAQW section
"The United States has copyright relations with most countries throughout the world, and as a result of these agreements,
we honor each other's citizens' copyrights"
And Check out
http://www.copyright.gov/circs/circ38a.pdf and as you can see, that list covers a lot of countries, including South Korea.
And Aftermath records once got sued for using a song without permission by a Indian (India citizen) composer for usuing a sample in the Truth Hurts song Addictive. The case took place in a US district court in LA. Then Saregama India Ltd a company based out of India sued in a Houston court over music that was used without their permission. And in both of those cases the Indian person and company didn't have any copyright filings in the US for their work. NCSOFT do. So yeah just because the company is not in the US, doesn't mean the stuff is free game and cant be pursued in US courts.
But there is a chance NCSOFT may not bother, but they may bother, but still they may not. I think it's best to assume they will and be cautious just in case instead of going free range assuming we are untouchable. Because usually those that think they are untouchable unsinkable infallible, usually end up touched, sunk, fallen. And it's not because they were stupid or wrong but simply because in their haste they overlook something important and made a mistake that they could have seen and avoided if they slowed down and did a proper risk assessment. AKA they usually cause their own downfall.