People who live in Pyongyang are selected, usually because of their connection to politicians. The rest of the country resides either in primitive villages or labor camps in the north, where they are barely clothed and fed during the harsh winters.
That reminds me, I read somewhere that people in North Korea tend to smile when uncomfortable.
In an amusement park, at a bowling alley, at dancing events, in the park, and playing in streams, it seemed like people smiled about as much as you would expect them to anywhere else. I encountered some who wouldn't stop smiling.
What always tends to happen with photos and conversations of certain things (e.g. DPRK) people tend to see what they want to see and manifest their own narrative. Tend to.
I'm very interested in North Korea and this is a fascinating album.
* http://instagram.com/dguttenfelder (n.b. more recent photos are Philippines, not NK)
https://sites.google.com/site/sophieinnorthkorea/home
Maybe it is to save on energy costs? I imagine most citizens are used to it. Furthermore, if they have never experienced indoor heating, they would not see anything odd about it being so cold indoors.
I do this all the time on website that do this cover trick.
You can see similar strips in the west around secure areas, for instance nuclear power plants or particular buildings in secure sites. Sometimes these are dog areas, or contain mines.
I suspect the inmates don't have android phones though.