On a more realistic note, it's for the same reason that Israel refrained from retaliation when Saddam Hussein rocketed Israel during Desert Storm (remember Hussein's invasion of Kuwait?). Had Israel counter- attacked, the entire Middle East region would have gone up in flames. Israel is seen, even in many Arab countries, (at least at the moment) as being justified in it's attempt to secure its borders, prevent further incursions and seek release of the kidnapped soldiers. jake is the closest is observing that the U.S. (nor the U.N., in Africa incidentally) doesn't always intervene, at least militarily (usually humanitarian, though). In this case, the U.S. and perhaps the U.K. proffer supplies, strategic information from satellites and other valuable but unobtrusive support.
I'd be remiss though, if I didn't point out that Nigeria, Libya, Algeria, Egypt, Angola, Gabon, Congo, Cameroon, Tunisia, Equatorial Guinea, the Democratic Republic of the Congo, and Cote d'Ivoire all are either producing significant oil or will in the near future. (75 billion, with a b barrels proven reserves). To my knowledge, the U.S. hasn't invaded or otherwise asserted dominance in the area. Granted, most of the oil exploration and devlopement funding is coming from U.S. and western sources... (along with significant funding for hospitals, schools and infrastructure)...