Think about it....Do you think there where ALWAYS coming into the US via US only flights?
They weren't. It makes a certain amount of sense to try to vary the ways you get to a certain place.
It seems as though you think that their only focus is using planes as a means to either get in or bomb another plane or use it as a missile again.
I am putting aside the planes as the only method/means to achieve their goals. It's very possible they will continue to find a way to bomb our citizens, in this country, regardless of a plane.
Look abroad; they love to bomb hotels. They don't need aircraft, every time, to transport either themselves or the bomb material to achieve their goals.