An interesting commentary by Ken Menkhaus, the doyen of US Somali academics, one passage says it all:
The Westgate attack is the latest sign of the group’s weakness. It was a desperate, high-risk gamble by Shabaab to reverse its prospects. If the deadly attack succeeds in prompting vigilante violence by Kenyan citizens or heavy-handed government reactions against Somali residents, Shabaab stands a chance of recasting itself as the vanguard militia protecting Somalis against external enemies. It desperately needs to reframe the conflict in Somalia as Somalis versus the foreigners, not as Somalis who seek peace and a return to normalcy versus a toxic jihadi movement.
Link:http://thinkprogress.org/security/20...n-desperation/

Note his view that the Somali desire to make money acted as a restraint on Al-Shabaab.