Good point. That makes sense. Tow ratings are always a bit opaque. I recall that Jeep Wrangler 4 doors were rated higher than the 2 doors, despite having same engine/transmission/suspension and the vehicle weighing more. Speculation was because of the longer wheelbase of the 4 door, it could safely handle more of a load - but that wasn't factory verified.
So in reality the CX5 was built for and the engineers deemed it capable of towing 3,500 #s max. Hence the reason they can tow 3,500 #'s in Europe with the same exact CX5 platform engine and transmission. The only difference is that in Europe the towing laws are different than in the USA.