HDMI is a digital interface, when sound is sent over it the sound is therefore also digital. To change the volume of a digital signal means completely altering the digital signal. Apple appear to take the view that the end-point device i.e. the device actually outputting the sound should be responsible for volume control.
If your HDMI goes to an AV Receiver and the AV Receiver has the speakers plugged in to it, then the AV Receiver is responsible for converting it back to analogue (for sound is an analogue waveform) and therefore the AV Receiver should be also responsible for doing the volume control. It would be able to do this in the analogue form rather than the digital form since it will already have had to convert it back to analogue to play through the (analogue) speakers.
The same would apply to a TV with built-in speakers.
Now what is annoying is that even with the above there would still be a way for the Mac to send the volume control to the end device using a standard called CEC (Consumer Electronics Control). This allows sending such commands via the HDMI interface to be processed by e.g. the AV Receiver. The Mac would then be telling the AV Receiver to increase or decrease the volume, this would still mean the digital signal is unchanged and the AV Receiver still gets to do it in the analogue realm.
Unfortunately it is my understanding that Apple do not support the CEC standard although I have seen some people claim it 'works for them'.
For what its worth there is a USB gadget that can 'insert' CEC commands in to the HDMI signal, the computer would send the commands via USB to this gadget and the gadget then inserts the signal in to the HDMI signal. However the software your using has to support this gadget. I believe XBMC for Mac does so but iTunes etc. would not.
See http://www.pulse-eight.com/store/products/104-usb-hdmi-cec-adapter.aspx
I would guess any software like AudioSwitcher is converting the sound to analogue, altering the volume, and then if needed converting the sound back to digital to send over the HDMI, and then the AV Receiver (or TV) will have to convert it back to analogue again. As you can imagine this would make a small loss of quality and possible audio delays due to the repeated conversion. This is why I believe Apple don't do it at the Mac end. Although an AV Receiver generally also has the ability to set an offset to compensate for audio delays (TVs don't).
Again nothing would prevent Apple supporting CEC which does not have this problem.
