I believe the answer lies in that some types of batteries, especially if you have two bays full of them, would block the view of any item of similar composition below it. Meaning, if your laptop is lying flat on your bag, which is also flat, and a nice slim long knife is taped to the bottom of the laptop, they can't see it. Whereas, if you put it in the tub, well, they can see it. Just my $.02.