Because benchmarks of barbarity had not fallen far enough:

Insurgents in Iraq detonated an explosives-rigged vehicle with two children in the back seat after US soldiers let it through a Baghdad checkpoint over the weekend, a senior US military official said Tuesday.

The vehicle was stopped at the checkpoint but was allowed through when soldiers saw the children in the back, said Major General Michael Barbero of the Pentagon’s Joint Staff.

“Children in the back seat lowered suspicion. We let it move through. They parked the vehicle, and the adults ran out and detonated it with the children in the back,” Barbero said.

The general said it was the first time he had seen a report of insurgents using children in suicide bombings. But he said Al-Qaeda in Iraq is changing tactics in response to the tighter controls around the city.