Maybe I am way off your question here, and if so, I do apologize, soul patch.
Instrument Tracks and MIDI Tracks are not the same thing. Even though they both deal with MIDI, they are fundamentally two different things (hence the two types of tracks). An Instrument Track is tightly coupled with its VSTi whereas the MIDI Track is only loosely coupled and do not know what it is routing to (could be a VSTi in Rack or to a MIDI Port for an external device). This disconnect is probably the reason why it is the MIDI Volume versus the Audio Volume. I think the reasoning is that the Instrument Track is so integrated with audio that MIDI volume is really secondary to audio volume.
Not the answer you were seeking, and maybe I misunderstood your question, but there it is. 
And oops, Steve beat me too it!!