Because of actuary tables, and if they don't rate it you MAY use your wife's or husband's larger truck to do all your towing. And in Europe many have only one car at best, and I would guess they don't sue as much.
Plus an added note. When I owned a Mazda 5, I was at the dealership and was talking to a sales guy and he said they where looking out the window one day at the car lot and they saw a guy pull up with a Mazda 3 with a Bobcat on a large trailer. He walk into the parts dept to buy a part and drove off. That is why they don't want to push trailer towing. Because of the stupid. We both assumed the guys truck broke down and he had a "job" that had to get done and he used his other car to make it happen. Est. of that weight on that setup would be in the 7,000 lbs plus.
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