I was wrong.
So I figured out where I was wrong through some experimentation. Apologies for adding wrong information, and confusion, when I was sure based on my receiver's input signal and a brief few moments with a streaming 5.1 channel source that I was getting mutli-channel audio from SmartHub. It certainly appears to me that the SmartHub apps are, as you and the customer service reps indicated, only passing on a 2 channel source.
I ran the AIV video again, and again confirmed that my AVR was receiving a 5.1 channel signal, and that in unprocessed audio mode was putting out 5.1 channels. "What the hell?" I thought - how can this be? After futzing around some and puzzling some, I ended up in the Samsung audio menu, and saw that the SPDIF output was set to DTS Neo 2:5 (have no idea how that got set, since I don't remember playing with that, ever, but maybe I did or maybe my son did when playing xBox). But long story short, the Samsung app was apparently taking a 5.1 channel feed, down-mixing to 2 channels, and then the TV's audio processing was upmixing via DTS. So my receiver was in fact receiving a 5.1 channel input and displayed the same, but it was a processed signal from the TV itself. With the TV's audio output setting at PCM (jnstead of DTS Neo 2:5), the receiver is only getting a 2 channel input.
I fully agree with you that this is just plain silly -- there is no reason that SmartHub apps should be downmixing a 5.1 channel source. Terrible.
I consider this a pretty serious shortcoming in SmartHub, and will register my complaint with Samsung, Amazon, VuDu, etc. as well. Just plain stupid.