Here's an interesting link.

Apparently KLM is joined by Air France and Lufthansa which also overfly North Korea. Seems even more strange as they just fly over a small North-Eastern portion of North Korea, which can be avoided by a very small detour further East.

Also found a Facebook-link with a question about this issue to which KLM replies as follows:

Hallo, Jacqueline. We vliegen op dit moment nog gewoon over Noord-Korea, er is geen sprake van dreiging. Uiteraard houdt KLM dagelijks de situatie goed in de gaten. Bestemmingen waarbij over NK wordt gevlogen zijn Osaka en Fukuoka in Japan.

translation: "Hi Jacqueline, we are overflying North Korea as there is no treat. We are monitoring the situation daily. We overfly North Korea on our way to Osaka and Fukuoka"

I will be writing to a member of parliament to alert them to this issue.