There's absolutely nothing in most jurisdictions that require corporations do seek to maximise value without other considerations and at all cost.
Even if that was the case what "maximizes shareholder value" is highly subjective and/or hard to assess objectively in most cases.
E.g. if you believe (whether true or not) that equalising corporation tax would benefit the finances of your customers (maybe your customers are mostly small cap domestic companies that are damaged by competition with large multinationals that can afford tax sheltering; or maybe you think making such tax sheltering impossible will encourage companies with large cash hoards abroad to actually start spending, since paying tax on the profits means even low returns from reinvesting in expansion will suddenly be relatively more attractive than it used to be), then you might well have a reasonably held belief that such a tax would benefit your company.
I understand this statement to mean "Corporations have a fiduciary obligation to maximize shareholder value [within the law]".
Do corporations have a fiduciary obligation to change the law, as in your interpretation?
A lot of laws are essentially written by corporations for corporations.
Raising taxes on your profits is surely not in the interest of your shareholders.
It doesn't matter if it isn't worse for you competitively. It's worse for your profits.
If you could have more competition AND more profits, you would want that.