The libertarians constantly spouting that CEOs have a fiduciary obligation to shareholders are making the best argument for increased government regulation possible.
A reasonable libertarian however would require that all externalities be paid for (under the principle of doing no harm). If the price per unit of oil includes the cost of all the negative externalities (e.g. selling that unit comes with a requirement to remove the air pollutants and CO2) then it isn't really a problem (at least for the purpose of this discussion) if CEOs have to seek profit for shareholders.