Is it just a coincidence that the United States only ever seem to defend the freedom of oil rich countries?

There seems to be a pattern developing here - Iraq, Kuwait, Egypt and Libya. The United States seems to be very eager to defend the "freedom" of these nations. How come the United States doesn't seem to show any interest in defending the freedom of other African countries whose people live under the yoke of corrupt and evil dictators and aggressors? Is it because these countries aren't rich in natural resources?