Currently, companies decide what is "right" and "ethical" using a standard - shareholder return - that has been proven time and time again to have unethical consequences.

Not infrequently the consequences are literally disastrous and/or fatal. (See also Boeing, Lehman, Enron. HP at its worst, and many many more.)

What exactly gives corporations the moral authority to act like this?

Is it simply that large accumulations of money somehow render other moral concerns irrelevant, as if by magic?