Because the people responsible for the mess had already jumped ship, escaped the repercussions, and the ignorant populists demand blood, even if that means stripping away the "bonuses", which are effectively salaries as their base salary is near $0, for the remaining people that have had nothing to do with the misdeeds of their predecessors.
There was massive incompetence and destructive greed at AIG and that can not be disputed, but the reckless demand for "justice" is not accomplishing anything.