The company doesn't exactly have a fiduciary duty to you. It (or more specifically, its agents) have one to the company itself. This can be broken in cases of fraud, illegality, or conflict of interest. For example, in Caremark and Trans Union, the directors were so checked out that they should have known better--you can't sell a company for a random value picked out of a hat.

Beyond that though, the business judgement rule is supposed to protect against second-guessing plausible decisions.