It was probably intended to prevent the export of cheap korean games to other countries At the time they probably did not anticipate that the key would be found and incorporated into cios. When the korean wii was released it was very easy to load out of region games on any wii via modchip or loader, so they probably wanted to prevent the export of these games possibly. It is the only thing that makes any sense since the korean wii has the common key and can load any game from any region, but all other wii's can not load korean games. Possibly these games have some korean propaganda incorporated into them that the korean government did not want the rest of the world to see, who knows. It clearly has nothing to do with the piracy of korean games to korean wii's. Since korea is very restrictive of what media enters the country, perhaps Nintendo passed it off as an effective way of preventing western games from entering the country and influencing the political beliefs in an unwanted way. Certainly Nintendo would have known it would not work, but it may have been enough to convince the korean government to allow the wii into the country for sale.
What you say in the first part really rectifies the idea of anti-piracy measures. Of couse, all that you say is pure speculation and we can all speculate any idea about this, but honestly, nintendo built his console on thinking that it was secure. Maybe the crazy korean government required more for security measures that caused them to only release a wii conforming to that region a few years after it's initial launch, but that is entirely stupid to think that it took them multiple years just to work with the korean government when they already thought it was all secure.
The fact of it is that they know that the korean region has a higher piracy rate than any other national specific region. Nintendo knew this and they spent extra time and effort to go above and beyond to thwart the likelihood of hacking. Security through obfuscation was the goal in this instance.
No one can positively come through and say exactly the reasonings for the korean key and the korean IOS's, but what I present is the most logical reasoning that there is.