Probably for the same reason most app piracy also comes from China and Russia. On iOS your competitors may be having paid apps, so if yours is free, they'll get yours, while downloading almost none of the paid apps from competitors.

The point is, people there can't afford to buy apps, so they'll get them for free (either from the store, or pirate them). Both countries have large populations, too, so you're more likely to notice the "surge".