rotorfan:
…it's complicated… as they say in Facebook.
Lots of reasons for Vne - rotor hub stresses, mast twisting moments, retreating blade stall (and margin for same) - for example, did you ever try an emergency evasive maneuver (avoid flock of birds) in the cruise in some popular machines? You might be surprised at how close to something unpleasant you are.
But nearly all helicopters have a reduction in Vne with increase in density altitude due to TAS effects - whether it's for the advancing or retreating side is not up to us to argue. We have only to obey the limits.