There is a premium in the South Korean exchanges because it's hard to arbitrage.
Normally, if say Bitstamp was trading higher than GDAX, people would sell coins on Bitstamp and buy them on GDAX, and as a result the price dfference would narrow.
It's impossible to do on the Korean exchanges because they don't let westerners join or send fiat to those exchanges.
That is a good point. I just don't know why the government would not let foreigners join because that created this mess.