Afaik they drop usb sticks with south korea tv shows / american tv shows / chinese tv shows over NK because a lot of people (especially in the border region to china) have dvd/usb player even if illegal and risk of concentration camp for their family + 2 generations of their family.

https://nypost.com/2017/03/17/importing-hope-into-north-kore...

https://flashdrivesforfreedom.com/

http://www.sueddeutsche.de/digital/datenschmuggel-geheimwaff...

https://www.theguardian.com/world/2016/mar/22/flashdrives-fo...

Thought that was common knowledge nowaydays.