HBO requires a level of content protection that’s not currently supported by Apple TV.
If you recall, I bought the iPhone HDMI adapter with intentions of streaming HBO GO and came away disappointed. Likewise, even AirPlay Mirroring, from iPhone 2 to Apple TV, is blocked. A couple months back, the logical assumption would have been that HBO prefers folks subscribe to their cable channel and isn’t interested in digitally serving the lean back crowd. Or they were concerned with various content licensing issues. However, after announcing intentions to stream HBO GO via Samsung devices, the Xbox 360, and Roku those theories have been blown. And, now, it looks like we have our answer – there’s something about the way Apple transmits data via HDMI and/or AirPlay that makes HBO uneasy. But with HBO GO coming to various set-top boxes (that I own) in the near future, it doesn’t much bother me anymore.