...there is little reason to overthrow these governments to get their natural resources as the US (or anyone else) can buy them for shiny guns.

No doubt.  Oil being fungible, I always understood the US wanted control over the resources instead of appropriating itself of them outright.

After absorbing the information from the commenters here, I believe I have answered my own questions.  I conclude the US is not intent on taking over what appears to be a limited reservoir in Somalia.  I take the US's paranoid claims at face value:  It is all about preventing Al Qaeda from getting a foothold.  But that does not negate that an underlying reason has to do with the overall security of oil routes, spillover into neighboring countries and possible impact on the wider middle east.

Furthermore, I agree that US and Ethiopian actions (aside from being illegal under international law) might have the opposite effect of fomenting (instead of discouraging) radicals.  Kind of like Iraq:

The Prime Minister of Ethiopia, Meles Zenawi, must have been studying the magnificent successes of the U.S. preemptive invasion of Iraq and Israel's recent foray into Lebanon. He has clearly decided to emulate them. His argument is exactly that which was given by George W. Bush and Ehud Olmert. We must attack our neighbor because we have to keep Islamic terrorists from pursuing their jihad and attacking us.

In each case, the invader was sure of his military superiority and of the fact that the majority of the population would hail the attackers as liberators. Zenawi asserts he is cooperating in the U.S. worldwide struggle against terrorism. And indeed, the United States has offered not only its intelligence support but has sent in both its air force and units of special troops to assist the Ethiopians.