This stood out to me in the article, as most countries incentivize foreign visitors to use local currencies to boost its strength. However, NK seems to have long ago come to terms it's currency will never hold value, so perhaps they instill this rule to boost their counterfeit money operations, or reserves of foreign currency.
They have stolen $3B+ over the past several years alone.
https://therecord.media/north-korea-cryptocurrency-hacks-un-...
You used to be able to get a set off eBay for like $25, and sometimes uncut sheets of the older series, but I haven't looked in years.
Fun fact: it's probably easier to collect a reasonable type set of North Korean notes than South Korean. The Southern collection has a lot of hard roadblock notes-- a lot of notes that were replaced by the end of the 1960s and 1970s, so quality examples are expensive and rare, and one famous type from 1962 that circulated for less than a month before being flash withdrawn.
Plus, many products in the shops for local are probably heavily subsidized and have vastly lower prices.
If foreigners can only own foreign currency, and normal citizens can only own local currency, then it makes it a lot harder to bribe the locals!
The three-tier model was a little but unusual, but the idea of a secondary currency for foreigners was not that uncommon.
One other reason for it is to ensure "minimum spend" -- some countries required you to buy a certain amount of the foreigner currency at the official exchange rate as part of the entry process, even though the black-market rate for "local" currency was far more competitive.