seems like this have been discussed before.
http://www.airlinepilotforums.com/fl...-approach.html

I see your point in regards to PIC/SIC, but both those pilots are legal to fly in IMC conditions as they are both instrument rated.
A ppl student is not instrument rated, and not allowed to fly in IMC (unless you're with him), I feel this is where the difference lies


edit:
Since you are actively giving instruction - and not acting as a safety pilot anymore, as you are in IMC - you are performing the approach by guiding the student in the same way that the student could perform the approach by telling the autopilot to fly it, and you're doing it in actual instrument conditions per 61.51