Chris Christie wants to track all visitors to the US all the time, so as to find them if they overstay their visas.

How could they do this? Implanting a portable phone into the visitor's body would work. As a bonus, they could listen to all conversations that take place near the visitor.

Doing this to visitors is a great wrong, but it would not remain limited to visitors. It would subsequently be extended to US citizens the next time some spectacular crime committed by a citizen provides an excuse.