The 5.1 does not pass from HDMI input X to HDMI (ARC) because of HDMI specification:
When an external device and the TV are connected directly, they negotiate their capabilities during initial handshake (resolution, copy protection, etc.). At that time, the TV tells the source that it is a 2CH device, because it is stereo. In response, the source (Xbox360, PS3, Cable box, etc.) downgrades its own audio output to 2CH. Then, this 2CH audio pass through the ARC and Optical.
In short, the TV does not down convert the audio. It just passes it. The HDMI source device down convert, because it was notified by the TV that only 2CH are available (the TV's own speakers).