Think about it: The breakaway switch does absolutely no good if you hook it to the receiver, and the receiver happens to come off the truck.
While it's not likely that the receiver would fail, it is so simple and easy to attach the cable to something that is not the receiver, that it makes perfect sense to attach it anywhere but the receiver.
It does not take much force to pull the breakaway pin, so whatever you connect to does not have to be particularly strong, just solid and not likely to fall off the truck any time soon.
I attach my breakaway cable to the truck's bumper. I use a 300lb rated carabiner and clip it through a slot on the underside of the bumper.