"Best interests" is deliberately vague. Is it in their "best interests" that you fire the CEO and call the cops because he raped an intern? Or maybe that you agree to accelerate his $40M "bonus" if he agrees to resign without saying why? Or maybe it's in their "best interests" that you pay PIs to develop evidence that the intern has a drug habit and "leak" this "shocking revelation" to the media if they go public?
You can argue almost anything meets this criterion, in some egregious scenarios a court won't buy it, but they will give you enormous leeway.
People write this sentence seeming to imply it means "CEOs and management have a responsibility to be as ruthless and sociopathic as possible to deliver the highest returns, and any consideration of the people or communities they trample beyond legal requirements is itself borderline illegal."
There is a lot - A LOT - of room for ambiguity and debate on the specifics of shareholder value and "best interests." The "legal constraints to act in the best interest" is not some set of corporate rules and KPIs codified into our legal code write large. It's not about maximizing a specific KPI over a fixed timeframe.
Not to mention Sweeney is the majority shareholder in Epic's case.
Businesses are legally bound to act in the best interest of their shareholders. This is quite an open ended precedent.