In the U.S., there is no fiduciary duty to minimize tax liability or to maximize profits. In any case the bar for a successful shareholder suit is extremely high, and executives are given tremendous leeway in situations where conflicts of interest aren't involved. I can't imagine that you could successfully sue a CEO for a company's not taking aggressive tax positions.
Achieving low tax rates is largely about keeping up with Wall Street analyst expectations. CEO's of public companies having to face a bunch of analysts every quarter to discuss quarterly earnings, including after-tax earnings, is a huge driver of this sort of behavior. CEO's are judged on how analysts view their stock, and it's a horse race. If some other company makes $0.05 more than you per share because of a lower tax rate, they are going to get credit for that.
There is a wide band of freedom given to business judgement. The critic that is the capital markets is a bit more harsh, however. If your returns aren't good enough relative to other investments of similar risk because you opted to pay more in taxes, you may have a harder time attracting investments. That's the theory at least, and a pretty good one if you ask me. To ignore that is to basically ask for not only increased taxes, but increased cost of capital overall.
