They think all laptops are potential bombs (but somehow iPods, phones, video players and other electronic devices are not

)
So their conclusion is putting the laptop through on its own makes the 'bomb' easier to see - of course given they can only detect 20% or so of the bomb or bomb parts they are tested with, I'm not holding my breath.
Just another silly rule - although certainly not on par with the stupidity of putting liquids/gels through the x-ray on their own because the TSA believes the x-ray can detect the chemical composition of these items - now that rule is unbelievably stupid.