Maybe it was a way to prevent 'real' money leaving the country, or the only way to buy North-Korean money abroad. I don't think the people of Pyong-Jang really cared if the notes were spent by 'believers' or 'non-believers', but by looking at the banknotes, a lot of privacy of the bearer was given.
In this 1978 set, we see red serial numbers, North-Korean inhabitants used notes with red and black serial numbers, other socialist tourists had banknotes with only black serian numbers. As this shown set has only red serian numbers, so for 'kapitalist' tourists only.
|Front North-Korea n°18, 19 and 20|
However, at the back, some details are important too...
The notes for 'locals' with red and black serial numbers were replaced by notes with red serial numbers only, but can be distinguised by the 'non-socialist' notes by a number in a red guilloche on the back.
The tourist notes have a seal on the back, but no number.
So let's have a look at the same 3 notes that at first appeared to be 'non-socialist, tourist notes' :
|Back North-Korea n°18c, n°19d and n°20c|
|red seal on n°18c|
|red seal on n°20c|
|red guilloche on n°19d|
So this means, that the 1 and 10 won note were issued for western tourists, but the 5 won note was issued for local (North-Koreans), but is a replacement note.
|heroic family North-Korea n°18|
|labour and growing food North-Korea n°19|
|winged equestrian statue "Chonlllima"|
In this set, also a 50 won and a 100 won note were issued.
On the 50 won, the same serial numbers/seals and guilloches as in the notes above.
The 100 won note only came in the first emission (red and black serial number - local use)