I think we're mixing up terms when we talk about cargo capacity and towing capacity.
cargo capacity is the amount of weight that is supposed to be on your vehicle's 4 tires, period. for my SUV, that's 860 lbs., inluding people, gear, and tongue weight.
As I understand it, that means the trailer weight only matters as it pertains to how much pressure is put on the tongue (i.e. pushing down on the car's chassis and tires).
so I can pack 860 lbs of 'stuff' IN the car, minus my tongue weight. The towing capacity remains constant, as long as you don't exceed the tongue weight; or rather, I would think they assume 100% cargo weight when calculating towing capacity.