In all probability, most of the "tourists" are simply old S. Koreans attempting to visit their N. Korean relatives (brothers, sisters) for a few days, hand over a few "CARE" packages, and then leave without actually "touring" the country.
I suppose that, what with N. Korea now having been isolated for more than 60 years, such family reunions are becoming more and more seldom.
Regards,
When I lived in Hong Kong you could take guided tours into North Korea - as long as you were Chinese looking
Evidently there are a fair number of people that go there