What they are getting at is the concept of "fiduciary duty". Executives appointed by a board or partners in a partnership must act with fiduciary duty, which generally means they have to act faithfully to the wishes of the shareholders. BUT, the shareholders can specify any values they wish. They don't have to be maximizing shareholder value, they can be to pursue environmental goals, or social improvement, or anything they so chose.
In modern publicly owned corporations, most shares are actually held by intermediary banks, and votes are done via proxy. Most shareholder resolutions fail, and votes are given to those who most claim to maximize shareholder value. It doesn't NEED to be this way, but this is the way it is now. But it could be different.
They're required by law to follow their corporate charters, and those almost always require maximizing shareholder value. They don't typically restrict that to short-term value though, so long-term growth strategies aren't ruled out by this.
> Corporations are required by law to maximize shareholder value
No. And saying it more often does not make it any more true.