I have to disagree a bit here. If I buy a trailer, with a given weight rating. I expect it to be able to carry that weight with any tow vehicle using only industry-standard components. I might only plan to carry a small tractor at this point, but there is no reason I should have to tell the dealer everything I might want to do in the future.
What if a neighbor moves to the city and offers me a screaming deal on his 5500 lb. tractor? I can't possibly know this is coming, but if I have a 7000 lb. trailer, why should I have had to know about this when I bought the trailer?
The manufacturer is supposed to know that when he rates his trailer, that every truck in the US is going to need a WD hitch to carry that load. Making a trailer that can't be set up to carry the full rated load with the trucks available sounds just plain fraudulent to me.
If you can't use a WD hitch with the trailer it should be de-rated to 5000 lb. max load, because that is the practical limit. And, I bet I could buy an all-steel trailer rated at 5000 lb. which would weigh less than this aluminum one, have a higher payload, and cost less to boot.