[Grimm1]

It hasn't not been legal for a while.

https://www.lawschool.cornell.edu/academics/clarke_business_...

"Third, corporate directors are not required to maximize shareholder value. As the U.S. Supreme Court recently stated, "modern corporate law does not require for-profit corporations to pursue profit at the expense of everything else, and many do not do so." ( BURWELL v. HOBBY LOBBY STORES, INC. ) In nearly all legal jurisdictions, disinterested and informed directors have the discretion to act in what they believe to be the interest of the business corporate entity, even if this differs from maximizing profits for present shareholders. Usually maximizing shareholder value is not a legal obligation, but the product of the pressure that activist shareholders, stock-based compensation schemes and financial markets impose on corporate directors. The Shareholder Value Myth , Eur. Fin. Rev. Lynn Stout (April 30, 2013) The Ideology of Shareholder Value Maxim (Watch), Evonomics"

[chipotle_coyote]

AFAIK, modern corporate law never required that, and the Supreme Court was affirming the existing state of things. The "Shareholder Theory" stems from an essay Milton Friedman published in 1970 asserting that corporations have no responsibility to do anything other than maximize shareholder value, but this was never enshrined in law or financial regulation -- it was just something a lot of corporations followed. It seems in the intervening decades it's become such an accepted "truth" that people assume that it's a legal requirement.

[Grimm1]

Yup and it was then largely popularized by Jack Welch former CEO of GE who is/was responsible for a lot of modern execs views on the matter.