I don't know about IOI but there were few incidents with North Korea on IMO. I have heard that one time they cheated (oh, really?) https://www.quora.com/In-the-32nd-and-51st-IMOs-i-e-in-1991-and-2010-how-did-the-IMO-jury-conclude-that-North-Korea-had-in-fact-cheated and another time one student successfully fled and didn't return to North Korea (http://www.scmp.com/news/china/diplomacy-defence/article/2074200/n-korea-maths-whizzs-long-hong-kong-ride-freedom)

I'm not an insider, but apparently it is very costly (not in economic terms but..) to bring their most gifted young students to an international event. So the party had to make a choice, which was IMO. (Note that they didn't send their teams to IMO 2017)

And I doubt that pre-univ students in NK has free internet access to study competitive programming.

Because if they don't win gold medal they will be dead

Because they are busy making missiles.

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