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Actually, they have a fiduciary duty to act in the interests of the company, albeit that if the company is solvent then the interests of the company are generally viewed as being the same as the long-term interests of the shareholders. However, directors can easily claim that the long-term interests of the company do not require or justify immoral conduct. Equally, they can hide behind the duty to act in the best interests of the company if they're weasels, and want an excuse for being weasely.
Well, they have a fiduciary duty to act in the interests of the shareholders. It is up to the shareholders to instruct the directors about their interests, but in most cases maximising value is at the top of the list.