For the most part, the title says it all. Instead of passing through stereo from the source (netflix), the Stick sends left and right signals over 5.1.
Others have complained of this issue on Amazonforum, avsforum, and reddit:
When I watch old shows on Netflix, such as Cheers, or Star Trek Voyager, being able to enable pro logic would mean moving the bulk of dialogue to the center channel, making dialogue much clearer and seperate from sound effects.
This isn't even a "feature" that would be nice to be added, but rather, a basic, necessary functionality of a streaming device.
Why devs didn't just let the Stick pass through the audio unchanged, rather than processing it to no gain (there isn't a single benefit in handling it this way, no compatibility boost, no quality boost, nada) is completely beyond me.
I may just end up returning my stick (just bought it today). I don't know if this would ever be fixed. Previous versions of the stick didn't do this, so I'm not sure why this one does.