It's because an auto can get a load moving without relying on a clutch. The clutch is the weak link in manual.
When I first learned this fact, I didn't believe it either, because I always thought a 4 sp. manual with a granny gear would tow the most, all other things being equal.
I never really understood the manual thing either.
So I keep riding the clutch while towing and it goes out. I'd rather replace a clutch than an automatic transmission.
I think it is just so the manufacturer doesn't have to pay for new clutches during the warranty period for folks that ride it too much. They couldn't care less if you burn up the auto trans after the warranty is over.
One thing some folks dont heed is taking the auto transmission out of overdrive. You should never tow anything heavier than a utility trailer in overdrive IMO.