Al-Shabab Attack on Westgate Shopping Centre: Religiously Motivated?

Today, David Cameron has said of the attack on the Westgate Shopping Centre in Nairobi, Kenya "these appalling terrorist attacks that take place, where the perpetrators claim they do it in the name of a religion; they don't. They do it in the name of terror, violence, extremism and their warped view of the world."

Here is a link to the story so far, with an interesting paragraph towards the end which states that Al-Shabab, the organisation claiming responsibility for the attack, freed Muslims as the attack was only aimed those of a  non-muslim faith. 

http://www.bbc.co.uk/news/world-africa-24193059

The BBC news website also provides a good summary of who Al-Shabab are:

http://www.bbc.co.uk/news/world-africa-15336689

I'm just wondering who agrees with David Cameron? If we go by the social constructionist definition of religion, then surely Al-Shabab themselves fit the description? If Al-Shabab are carrying the attack out in the name of their religion, because that's what they feel they need to do as it is a meaning they have given to their religion themselves, then a social constructionist definition would say we cannot argue with that. Can we also justify it from a Functionalist's point of view? It could be argued to carry out all the functions of a religion identified by Durkheim, Malinowski and Parsons. Even before we take in to consideration any of this, Al-Shabab originate from the Union of Islamic Courts and imposes a strict version of Sharia law in areas under its control. Does this make it a religion, or could it be classified as an arm of a religion (Islam)?