For towing, it is simply that there are few people that can properly feather a clutch well enough to get more than 3500 lbs. moving without burning it up. Nissan doesn't want a bunch of folks pestering them for new clutches because they tried to tow 5000 lbs. and cooked them. And the shock that a load like that can put on the driveline can make its way back to the engine. An automatic's (fluid) torque converter absorbs most of the shock.
For the torque numbers, that's the torque converter itself, it actually multiplies the torque a bit:
Link
Brent