Russia and China like North Korea acting as a buffer against the US foothold in the region. If the North and South eventually reunite then it would most probably be allied to America (unless for some reason the North ends up being the dominant player in the reunification). In order to accommodate Chinese concerns US troops would probably have to leave Korea for the reunification to take place otherwise they'll continue to prop up the North.

Or at least not have any bases in the former Northern Korea. China would not be happy about US military bases right on its border.

**If we ignore all the infighting, the North's invasion of the South, and the random attacks from the North to the South 

First off you are factually wrong about North Korea and secondly, I figured my statement that it worked so well in those other countries would be a blatant tip to others that I was not serious. Didn't realize people actually believed our meddling in those nations has produced positive results.

I can't believe someone actually took you seriously.

I didn't know North Korea was the most racist nation on earth-

http://www.slate.com/articles/news_and_politics/fighting_words/2010/02/a_nation_of_racist_dwarfs.html