MaskedShuuyu is right, it's really an effort to stop piracy more then blocking imports per say...
The import blocking tends to just be for the fact it's extremely simple to do on a CD, if they cared --so-- much about it, you'd see DS cartridges in different shapes like they did with the NES / Famicom. SNES / Super Famicom etc etc.
Like all things video game companies try to do, all it will do, at most, is delay things for pirates, boot-leggers etc.
..Remember for the PSP? The TA-082 motherboard emerged and it was supposed to be uncrackable? Hahahaha... So much for that idea.
Edit: I also recall, back in the day, one of the reasons why Sony mentioned they blocked imports (Talking the PSX days now..) was something that when people bought imports (Especially of games that --would-- come out in America, Europe etc) is that the nation that brought it out would be losing money etc. Example. If an RPG came out in America, and the European players wished to play it right away, they'd import it to save the .. what is it usually? An extra unneeded 8 months of wait? And when the game would come out, no one would buy their version. So, it was less of an exclusive thing. But, that was just what they said.
