The target audience is cord cutters with money that are willing to spend it. Why would HBO care about those that are mostly or only interested in 'free', HBO has never been about 'free'.
And the AppleTV numbers represent a very significant percentage of whichever market you'd like to mention that would have access to the new HBO service. Apple will do the billing from what I've read and you can bet like everything else offered on the AppleTV access to it will be dead simple.
But don't worry, it will be on other devices in a few months assuming it is successful.
While I've been in the middle of shifting things around for my TV needs, I got to thinking that if Apple would add Amazon Prime/Instant video to the AppleTV, they could take much more of the market. I know that if it was there now, I would be on the AppleTV much more than I am now.