The brakes on the vehicle help stop the trailer and this is designed into the vehicle. That's why trucks have such huge brakes for their size. Notice the brakes on trucks have gotten bigger over the years? Not because the trucks themselves are that much heavier, it's because they can tow much more than they used to.
The more weight you add to a given setup, the longer the stopping distance.
I have upgraded brakes on my truck and it stops the whole 17K# load faster.
BTW, I have seen both Ford and GM write about this very subject and they make it clear that the trucks are part of the formula and have a major impact on stopping the trailer. So yes, it is clearly part of the certification or more accurately, it's part of the design criteria.
