Well - first of all MUTE is a toggle function which means that sending out MUTE would un-mute devices that was already muted. Secondly, all telephones would be sending out the MUTE which would result in an unknown situation.
Sending out VOL_DOWN is also a solution but again all your telephones would be doing it and they might interfere with each other and (due to the range of the phones) might not reach the device at all. Also the phones would need to send VOL_DOWN to audio, video and link products. That would take a while... Executing an all_standby is also a bad solution for most costumers. Finally some costumers would dislike that their device was muted - imagine if it was _picture_ mute on a TV showing a soccer game where your team was about to score.. to somebody muting the audio would happen in unwanted situations.
So - no really good solution was found and it was left for the customers to do it manually.