Only for people travelling to the US?
I'm not going to discuss the merits of this particular suggestion but it does strike me as more than a little odd, given how easy it is in the US to buy munitions, that the restriction only be applied to flights coming into the US. All recent major terrorist incidents in the US were carried out by US citizens, though the number of deaths pales in comparison with RTAs and "ordinary" murders.
However, if I were a foreign national planning an attack on US soil, I would simply look at getting my supplies in the US and using some low-paid donkey (cleaning / luggage / security) to plant my device for me. This is how the most successful attacks are carried out.