I didn't want to turn that comment into a novel, but the reason they tell that story is because it's generally accepted as fact (among many other local border town residents) that the USA has means of listening to you speak in your car before you even approach the gate.
Well, the Canadian border agents were running an eavesdropping program until it became public and they supposedly stopped doing it. I think that if the canadians were doing they were only following in the footsteps of the US side.
