Sky+HD sound

You would think it’s simple.
Sky box has HDMI output. My amplifier has HDMI in with full HD sound and 7.1 decoding. So why does Sky box only output stereo sound through the HDMI cable? I have optical cable connected which does work fine ( DD 5.1 only). The installation tech told me the HDMI does do 5.1, he was wrong although it’s hard to find a definitive answer.
After about an hour of googling I finally found what I hope is the correct answer.

Why make a piece of kit that doesn’t do what it can? Is it that you can’t send HD sound over satellite signal? Perhaps they only have space for compressed Dolby digital rather than the more pure 48khz CD or better standard.
Sky need a tech page for people who know their stuff to help make decent equipment decisions.
Oh well.