In a classic "Seinfeld" episode, a distraught George runs over a pigeon and yells "We had a deal."
In other words, when birds fly in front of your car, there's no need to swerve. At the last second, they will get out of your way. No harm, certainly no mess.
The same can apply to driving courtesy with merging vehicles.
When traffic is heavy on back roads, there are certain intersections with a line of cars waiting to enter the main road. The "deal" is that courteous drivers on the main road will let one car out. That way, no one is left waiting.
Not two, one.
So when someone tries to piggyback on your courtesy, they are breaking "the deal."