I can presume since they were decent (not fanatic citizens) after the terrible loss of their daughter they might turn into fanatics against us? War on terror makes more people to voluntier joining terrorists? Never ending story....
Notice how his dad was not fond of the Al-Shabab at all, referring to them as fundamentalists to his daughter. Yet when his daughter was hurt and he sought help with the Al-Shabab soldiers, they disassembled the weapon and helped get her to the hospital. Contrasted with the Americans who just killed his daughter because it was an "acceptable" collateral damage, I bet you at that moment he felt much closer to the Al-Shabab fighters.