If I had to make wild guess, I would think towing something behind would put extra load on electric motor, which could lead to overloading and overheating the whole system. As far as I remember hybrid Highlander runs electric motor with 2 gear transmission: 1st gear for low speed, high load and 2nd for cruising at higher speed, lower load, therefore better suited for towing, which TCH doesn't have, so they're not exactly the same system. I would imagine towing something heavy with fully loaded car in the mountains is different than towing something light with only driver and no other load in the car on some flat road, so you could probably get away with towing in the good conditions.