Legal obligations to shareholders
"it suggests that their legal obligation to do the best by their shareholders over-rides just about any other concern."
It does not, and there is in fact no such legal obligation. The duties of the board of directors are ultimately to ensure the long-term success of the company, and while shareholder considerations are part of that, the shareholders' interests certainly do not override the prime responsibilities.
http://www.lawdonut.co.uk/law/ownership-and-management/the-board-of-directors/your-responsibilities-as-a-director-faqs#YRAAD2