(I'm not a lawyer so take this all with a pinch of salt.)
Fiduciary duty is not profit optimization. It basically means you need to be responsible with the company's money. You can't spend it on hookers (well, unless that's your companies business). It's basically a "don't waste money" rule, not "earn lots of money". Basically, the idea with corporations is that the shareholders own everything and the management is taking care of all the assets; it's not the management's so the management has to be careful not to waste money.
For example, legal guardians have a fiduciary duty.
> Dodge v. Ford Motor Co.
"The Michigan Supreme Court held that Henry Ford could not lower consumer prices and raise employee salaries. In its opinion, the discretion of the directors is to be exercised in the choice of means to attain that end, and does not extend to the reduction of profits or the nondistribution of profits among stockholders in order to benefit the public, making the profits of the stockholders incidental thereto. Because this company was in business for profit, Ford could not turn it into a charity. This was compared to a spoliation of the company's assets. The court therefore upheld the order of the trial court requiring that directors declare an extra dividend of $19.3 million. It said the following." (from https://en.wikipedia.org/wiki/Dodge_v._Ford_Motor_Co.)
"Among non-experts, conventional wisdom holds that corporate law requires boards of directors to maximize shareholder wealth. This common but mistaken belief is almost invariably supported by reference to the Michigan Supreme Court's 1919 opinion in Dodge v. Ford Motor Co.[5]
Dodge is often misread or mistaught as setting a legal rule of shareholder wealth maximization. This was not and is not the law. Shareholder wealth maximization is a standard of conduct for officers and directors, not a legal mandate. The business judgment rule [which was also upheld in this decision] protects many decisions that deviate from this standard. This is one reading of Dodge. If this is all the case is about, however, it isn’t that interesting.[6]"
> eBay v. Newark
"When eBay refused to sell, Jim and Craig deliberated with outside counsel for six months about how to respond. Finally, on January 1, 2008, Jim and Craig, acting in their capacity as directors, responded by (1) adopting a rights plan that restricted eBay from purchasing additional craigslist shares and hampered eBay's ability to freely sell the craigslist shares it owned to third parties, (2) implementing a staggered board that made it impossible for eBay to unilaterally elect a director to the craigslist board, and (3) seeking to obtain a right of first refusal in craigslist's favor over the craigslist shares eBay owns by offering to issue one new share of craigslist stock in exchange for every five shares over which any craigslist stockholder granted a right of first refusal in craigslist's favor. As to the third measure, Jim and Craig accepted the right of first refusal [7] offer in their capacity as craigslist stockholders and received new shares; eBay, however, declined the offer, did not receive new shares, and had its ownership in craigslist diluted from 28.4% to 24.9%."
" eBay asserts that, in approving and implementing each measure, Jim and Craig, as directors and controlling stockholders, breached the fiduciary duties they owe to eBay as a minority stockholder of the corporation."
I didn't understand this completely but it looks like craigslist was trying to restrict eBay's usage of the stock and that's a breach of fiduciary duty because eBay is a minority shareholder of craigslist and so craigslist is harming shareholders by their actions.