Activision Talks Region Locking as Free Markets Tumble

The real reason is simple: if you're the originator of a product, you can't get the best deal possible for your game if there's no region lock since anyone looking to license your game already knows that a good chunk of sales have been lost to importing and, thus, will either pay you less of a fee or might not license your game at all.

That's really it--anything else is just marketing/PR nonsense.