Political physics?

Warning: theoretical physics jargon follows.

Professor Nachtmann, who teaches the theoretical part of a course on the Standard Model, discussing the Lagrangian density of the free Dirac field:

"We can add a phase factor to ψ, which we can choose freely, since it is unobservable. However, we can see from the definition of the Lagrangian density that it has to be the same at all points in space(time); here, in America, and on the other side of the moon. Now this is of course highly undesirable: a theory where we could choose this factor freely at every point in space would be much nicer, since we wouldn't have to care what the Americans do."

Then he went on to show that the Lagrangian density for coupled photon and free Dirac fields does allow a position-dependent phase factor... For those who are interested in the physics: lecture slides can be found here.

P.S.: I'm curious how many of those who have read this understood the joke (to me it seems understandable with relatively basic physics/math knowledge, but I might be mistaken), and how many of those liked it...:p