Remember that under consideration is allowing Mexico to have some amount of land in Kansas as a sovereign Mexican territory to be used for a depot for transport trucks from Mexico. As I recall, the contents of the trucks would not be subject to customs inspection until they were leaving the depot for their destinations.
While this might pose a security risk, it would take a corrupt Mexican official to allow terrorist and/or terrorists gear to be loaded on the trucks.
As we all know, what are the chances of terrorists finding a corrupt Mexican Official?