I imagine that a lot of Blizzard's customers are kids who have decent graphics cards, and there's lots of them. Rather than get into all the trouble of dealing with washed/stolen funds in a currency that doesn't allow chargebacks, they could easily set up their own pool, sell BTC at the market rate to cover the cost of their games and stash a significant amount more as a future investment.

They wouldn't even have to publicly deal in BTC or even admit that they were doing it, just offer game credits for GPU time.