But this was true when Steam decided to start accepting bitcoin. There has to be some kind of profit motive behind making this decision at this point in time. I could only guess that they've forecasted a significant decline in bitcoin's value, whether it be the value of the coin itself or the costs of doing business with it.
They don't hold the bitcoin, they use a payment gateway who processes it into dollars instantly.
They stay clearly why they're not supporting bitcoin, it's because the time that it takes to mine the blocks means that often the transaction is processed after the payment gateway times out. And the mining fee is too high to make it viable for consumers.