Why did the al-Shabaab Islamist terrorists separate Muslims from non-Muslims before killing the non-Muslims?

David Wood of Answering Muslims has his theory as to why whilst Western leaders and the media assure us that the recent terrorist attack at a Westgate Mall in Nairobi, Kenya has nothing to do with Islam, and that the terrorist group al-Shabaab is violating the commands of Allah and Muhammad, opening the Koran shows us otherwise. He also believes it's clear that Islamic history has been repeating itself for fourteen centuries.