I'll bite, which law enshrines "corporations must deliver maximum return on investment" or whatever roughly equivalent idea?
A couple salient points:
The "best interests of the company" need not, and often are not, simply maximizing quarterly profits, share price, etc. Indeed, sometimes these things are odds with one another.
Moreover, shareholder value includes but is not limited to return on investment. It is simply most convenient _from a managerial perspective_ to simplify diverse shareholder notions of value down to the most legible lowest common denominator.