It is a given JAR25 requirement. VC is a design speed NOT an operating speed. VD is the design dive speed. If you go there the wings will come off. VMO/MMO may be equal to, but may not be greater than VC (the design cruise speed)
The reasoning behind that is, that if you are flying at VMO/MMO, and you get a disturbance nose down, the speed WILL increase, hopefully you will be able to recover before VD.