They don't fly over North Korea. The U-2S, RC-7s, RC-12s, RC-135s etc stay in South Korean airspace. The North Koreans have the missile capability to down a U-2 the same way as the Chinese and the Russians have done in the past. The recce flights stay in international airspace - far too vulnerable to go risking crews in this manner when the job can be done from stand-off sensors onboard.
The US almost certainly uses UAVs in particularly hot areas-that won't cause risks to anyone.
That's interesting stuff. I'm not really up on the latest tech.