Only the overt military operation is (sometimes!) frowned upon, and only if the government mismanages the spread of information so that the media gets a whiff. The US is very much in the business of conquering other nations through its economic dominance and puppet government policies.
Don't believe me? Think about who's really in charge in the US. Why does the US pursue an aggressive foreign policy at all, and why does it push for democracy in certain countries but not others? What are the benefits of fighting a war in Iraq? Clearly it's not so that we can make the world a better place, but it's also not just for the sake of "oil" or of "getting revenge". Who put Saddam in charge, who expected him to become a lapdog of the US, and who miscalculated that and must now go back once again to depose the rebellious general?
Others have more eloquently outlined the imperialism of the US, but it's not until we juxtapose it with that of a far more overt power player, in this case China, that we understand the differences of policies and why the US does what it does. Simply put, it's a matter of national image. China will need to work very hard to dispel its oppressive system image in the eyes of nations, but the US can simply be let off by the excuse of a dumb president catering to conservative values. When the next election rolls around and the US happens upon a "better" candidate, all will be forgotten and forgiven.
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Meaning you frown apon it..................POINT PROVEN