Well if you think about it, it makes sense. Something could be potentially improvised by a passenger. But to make a device, you would open yourself to detection via X-Rays. However, I think that the rest of the world should return the favor and ban all laptops from US incoming flights. Just as a quid pro quo.

Soon though hopefully solid state cells will come into play and this will no longer be a problem. China for instance has banned shipping all lithium ion batteries via China Post and searches actively for them... as it's a risk. You have to ship them specially declared which is pretty expensive.