Just spoke to my friend at Bell, who, he said, learned this 16 years ago from the program test pilot, test engineer and supervisor for the 407 testing program. Vne on the 407 is a function of measured mast bending during the test program and how that relates to component durability. That is the reason gurney flaps were added to the horizontal during the development program, so the current Vne could be reached with less mast bending.
Retreating blade stall occurs at a higher speed.
The Vne also prevents some possible roughness that could occur in very cold temperatures at higher speeds.
The 2,000 fpm climb restriction relates to possible mild control reversal at slow airspeeds, and high climb rates, where the horizontal is less effective.