Since it is well documented that the largest portion of the gas guzzler market does not have any legitimate practical need for gas guzzling vehicles, it is much more likely to be beneficial to society, on the whole, to phase out gas guzzlers. Potential injustice would also be avoided with respect to those individuals who do have a legitimate requirement for vehicles often considered "gas guzzlers." The proposal does not suggest imposing any ban on gas guzzler production and to whatever extent a genuine market for those vehicles exists among the minority of gas guzzler owners, general economic principles would guarantee that automobile manufacturers produced enough units to maximize the profit margin from any remaining market for gas guzzlers. If the proposal deterred everybody else from purchasing gas guzzlers, that would satisfy both Utilitarian and Rawlsian principles by virtue of the greatest objective benefit to society (Shaw & Barry, 2007).

Is it ethical for the government to essentially prevent a U.S. company from producing a legal good? What ethical theory supports your position?

The government is the final arbiter of the distinction "legal" and "illegal" goods. Throughout American judicial and legislative history, new laws, regulations, and rules replace previous versions. Frequently, what is legal today could become illegal tomorrow by statute, and vice-versa. Once it has been established scientifically that continued reliance on fossil fuels and petroleum products for energy production is not in the best interests...

There is no basis for ethical argument, particularly to the extent the U.S. government's reluctant takeover of the industry comes with decision-making authority by virtue of its ownership position, and also because it involves only indirect incentives and not outright bans.
Finally, what are the possible ethical considerations of banning or restricting the developing world's access to the same cheaper technologies (fluorocarbons, coal burning plants, etc.) that made the U.S. what it is today?

First, the U.S. has no authority to impose environmental responsibility on foreign soil or to ban energy production choices in other parts of the world. However, in principle, Utilitarian Rawlsianism would support any encouragement or political pressure to convince foreign governments to avoid making some of the same mistakes we now wish we had recognized and taken more seriously decades ago. To the extent that effort is genuinely intended to benefit the citizens of those nations and the entire world rather than just American interests, the Utilitarian/Rawlsian approach would support that effort; to the extent the effort is motivated by selfish concerns, it would not be justified (Shaw & Barry, 2007).