I'm sure there is a valid technical reason for this, but I dont know it:
I recently purchased a Samsung LED TV (7000 series) that appears not to support anything other than Stereo for components plugged into its HDMI ports. I have a Yamaha 401S (sweet) connected to the Samsung via the optical audio out port, and all other peripherals (ATV, Blue Ray, Xbox) connected to the HDMI ports in the Samsung. I know that HDMI cables can support multi-channel sound (at least, I know it can support 5.1) so why cant Samsung support passing that through on the optical audio cable? Is there a technical reason for this, or is this just a Samsung thing? Connecting everything to the Yamaha (much as I love it) and then Yamaha to Samsung is not my ideal solution...there are only 3 available HDMI ports on the Yamaha (excl the one that would be needed for the Samsung video), but there would be 4 on the Samsung.
Since your TV only has stereo speakers built in it has no reason to support more than two channels. There may however be a setting in the audio menu that will pass through 5.1 channels to the optical out on the inputs other than the over the air tuner.
To the best of my knowledge most (if not all) TVs will only pass 5.1 audio from the internal tuner. They don't pass 5.1 from HDMI inputs because of piracy concerns, since the optical output provides no form of "protection." It's frustrating since most devices with HDMI output also have an optical audio out, which if you really wanted to recorded the 5.1 audio stream you could just use that, so the limit on the TV output doesn't stop piracy but just frustrates the rest of us.