There are some airports in Europe like Amsterdam's Schiphol where you don't need to remove your shoes to clear security. If there is a bomb in the shoes of a passenger coming in a flight from Europe to America, then is obvious to say that all efforts made in America will be useless.
It would be like going to a concert venue, and having a metal detector on the doors on the east side while not using a metal detector on the doors on the west side. It is obvious that the person that wants to do harm will pick the west side.
It would be like inviting to a gathering at your house your next door neighbors and some other friends, while only requiring remove the shoes to your neighbors. The floor is going to get dirty obviously, the rule would be useless.
Can some one explain please?