It turns out that the people in red who were cheering for North Korea in their soccer game against Brazil weren't North Koreans at all, but Chinese actors. According to Hunter Stuart in the Huffington Post, the North Korean government paid them to show up and cheer.
It makes sense. If the North Korean government had let a few hundred North Koreans out of North Korea to go to South Africa, odds are that some of them would have defected from that Communist hellhole. Sure, the government could have threatened to kill or at least badly punish the families of the defectors, but they would still have had something else to worry about: that the North Koreans who were let out of their cage would see how other people live and would go back with those stories.
Here's what I wrote in a (still unpublished) article a few years ago, in which I referenced a story from Daniel Yergin's and Joseph Stanislaw, Commanding Heights:
Another such convert was the head of the propaganda department of the Chinese Communist Party. After visiting Japan in the mid-1980s, he wrote a report on his visit. He noted that half of Japanese households owned a car and that over 95% of households owned TV sets, refrigerators, and washing machines. What also hit him over the head was the variety of clothing people wore: "One Sunday we went out to a busy street. Of all the women we saw, no two wore the same style of clothes." Then he added, "The female workers accompanying us also changed clothes every day."