I am shook that North Korea is on that list and not South Korea.
It generally has to do with the nature of a country's economy. South Korea has a robust, diversified economy. Accepting of US dollars by a foreign country for transactions in that country is normally only common where tourism is the major industry or the economy is not overly diversified. This is especially the case when a large portion of the visitors carry or normally operate in US dollars.
North Korea is a different situation. Countries like North Korea that are isolated due to external embargoes or a closed economy have difficulty obtaining "hard" currencies, such as US dollars. They need those hard currencies to pay for any goods that they may need to acquire externally. As a result, they will typically accept or even require visitors to convert a minimum amount of hard currency for the local currency.
This was quite common in the 1970s and 1980s in eastern European countries before the Iron Curtain came down. For instance, in 1972 on the East Berlin side of Checkpoint Charlie, in order to get a day visa, I was required to convert a minimum 5 West German (BRD) DM for 5 East German (DDR) DM. I had to spend those 5 DDR DM while in East Berlin, since they could not be exchanged back. I was, of course, free to also spend as many additional BRD DM as I wanted while in East Berlin.
