I feel it is interesting they include this sentence:
"Exceeding these limitations may significantly reduce the performance
of your towing vehicle."
They don't define what they mean by performance, but we might assume they are referring to the ability to attain highway speed, maintain that speed in headwinds, perhaps an inability to maintain the speed climbing hills, suffer poorer fuel mileage or experience overheating.
To bring the discussion back to the specifics of this thread, my Frontier does not suffer excessively in any of these categories pulling either of my full height trailers.
Point being, that are NOT stating the sq ft limit is a safety issue, or even a excessive drivetrain wear issue. They only mention performance.
When I'm towing, I'm not in a race, so I don't mind having to run at slower than non-towing speeds, and I'm willing to accept the 8 to 12 mpg I get with the trailer back there.