Africa is awash with weapons. Somalia (Al Shabaab's home) has been a failed state for 20 years. The upper 2/3 of the country are basically autonomous. The Transitional Federal Government has barely managed to hold the capitol for most of its existence. There are 18,000 UN peacekeepers (funded by the EU) mostly from Uganda holding down the barracks in Mogadishu and selling their supplies for profit. One of Al Shabaab's biggest supporters is Eritrea, a nastly little country just up the coast which sends in weapons as fast as they can. Most African dictators give weapons to the rebels in their neighboring countries to make them unstable. Gaddaffi was famous for pumping arms into Chad and Sudan. The Somali army didn't get paid for several years so I'm sure lots of them sold their weapons as well. After all, the West will send more.
It does not seem like a problem that will be solved by the US toppling governments and providing more arms.