"What was wrong with simply re-instating a left turn from the lights for cyclists?"
Well - that would disrupt the free flowing traffic on Princes Street...
The no left turn (for all vehicles) is to allow there to be a pedestrian phase when traffic is going to/from Hanover Street.
'Of course' there couldn't possibly be a short left for a bike filter as it would reduce ped crossing time - fair enough (no chance of altering vehicle timings of course!)
BUT there is now a real dilemma.
Some people want Edinburgh to be 'more Continental'. In some places ped/cycle mixing 'works' without too much conflict.
So 'shared infrastructure' like this 'should' mean more awareness/understanding.
BUT another reality is that this (and some of the other current proposals) seem to be more about doing anything that won't reduce roadspace.