Earlier quoted context omitted.
There is an effect, because traveling itself creates chances for exposure. Anyone traveling on an airplane or train in close quarters, or passing through an airport or transit facility, can pick up or pass on an infection. By your last analogy, the red balls would have paint rubbed off them in the exchanges.
Yes, but then we should be canceling all flights in the US (we should be). Only canceling flights to Europe is a tiny fraction of what's actually necessary here.
When the concentrations in the US get higher it will start to make sense to constrain travel more over there. Right now the chances a domestic flight contains an infected individual is quite low, but that won’t always be the case.
Bear in mind the virus cannot be stopped, or contained at this point. All we can do is slow it down while also limiting the harm our countermeasures cause.