I'm pretty sure AirPlay anything is only stereo.
Edit: And even if AirPlay itself isn't, HBO is almost certainly only sending stereo, since the iPad's built-in audio output is only stereo. It would be silly to send 5.1 to an iPad, when 99.9% of the iPad's use is going to be with the built in speaker or headphones.
If you have an AppleTV, just use the AppleTV HBO Now app!
As for Netflix, are you maybe getting a Dolby Pro Logic signal? (i.e. it's digitally a stereo signal, but it has surround embedded in it - the way pre-digital systems did surround, like VHS tapes.)
I am definitely getting a full 5.1 signal using netflix (According to my ears, my receiver, and my remote - which displays the audio signal).
I thought that I had read somewhere that in some cases (supported apps, I guess), that Airplaying to the ATV would actually 'hand-off' the playing of the stream to the actual app on the ATV, thus rendering the ios device merely as a remote. This does seem to be case for netflix, but not HBO Now.
I am currently using the HBO Now app on the ATV, but honestly it is trying my patience in terms of the way it handles series that are currently being viewed. Not that the iOS apps are much better, but they are better - and touch screen makes for faster to navigation.
Unlike the iOS apps (and netflix), ATV app does not indicate which shows have already been watched. It does not provide an easy way to view the series currently being watched and which episode is up next. The Watchlist is a joke - especially if you have multiple people watching the same series (why in the world can you not setup user profiles?). And why the Watchlist functions completely differently between ATV app and iOS apps is beyond me.
Just found this quote from an old post (Oct. 2013) in another forum. Mind you the guy who made it only had one post, but this seems to be what is happening with Netflix:
"That is actually not accurate: for AirPlay Streaming (the feature just introduced by Netflix, as AirPlay _Mirroring_ has been supported since almost day 1), the content is eventually streamed directly by the AppleTV. There is no intermediate decoding/re-encoding step at the iDevice. Think of it as the iDevice being, quite literally, a remote control for playback with the AppleTV doing the actual playback."
I dunno.