Physical security is about making things more time-consuming/expensive to defeat or steal than they're worth. Scale the amount of concrete poured on/around and lock devices/armoring of such devices per local economics.
Definitely a good idea, but these were devices that were, by their nature, mobile. They had to be dropped for a couple weeks at most, then recovered and dropped again in a different location. Physical security would be a guy with a rifle and binoculars.
