That is a very broad and incorrect generalization. Some AWD systems work like that. Some are rear wheel drive until slipping, and some are true AWD where all tires are always connected at all times with a limited slip center differential. (Subaru, among others) Also, if you drive in an area with lots of icy roads, a good AWD system is way, way better than 4x4.
Many AWD systems have a viscous coupling in the "middle" (transfer-case etc) so that there's always power to the front and back.
Then to prevent wheel slip, the traction control computer applies brakes on any wheel that starts to turn faster than the average of the other wheels (which is why AWD vehicles might wear out the brakes faster when used a LOT on slippery surfaces) The system works very well. But it's interesting to note that NO manufacturer puts a system like on a 3/4T, 1T or larger truck
For 20 years, I towed a boat with my 1994 3/4T 4x4 Suburban and it did better than any 1/2T (empty) pickup would do because of the even distribution of weight. Pickups are simply not heavy enough in the back (Although I think my 4x4 F-350 Crewcab is heavy enough in the back)
No small boat will apply enough tongue weight either (that Searay 270 sure would though but I would NEVER try to tow one with a 1/2T)
IMHO, the ONLY good SUV's for towing (big trailers) are the 3/4T Suburban and the Ford Excursion(diesel or V-10.....The Ex was discontinued 10 years ago)
If you're going to tow a small boat/trailer, any AWD SUV might be just fine (as long as you don't exceed the GCVW rating) but if you ever want something bigger, you'll have to get a bigger tow vehicle.
I saw a picture of a Jeep towing a fairly big house trailer once that "jackknifed" The trailer was about 3 times longer than the jeep (and was probably double the weight of the jeep) Evidently slamming the brakes just picked up the back of that jeep and spun it around!!
If you want a truck, get a truck. If you want an AWD SUV, just make sure that the GCVW tow-rating is higher than what you're going to tow + the weight of the SUV.
Cheers,
Rick