To me and my pee brain engineering mind, It seems that the common sense answer here is that the Hitch itself is rated by the manf. to carry a certain load. as example 500 tongue weight vs 1050 TW using a WDH. This is the amount of weight that the manufacture of the hitch attached to the frame of the truck is certified to carry. Nothing added to or subtracted from the bed of the truck will affect that certification.
Now along comes the WD hitch Manf. and he says to select the proper WD bars you must add the load behind the rear wheels to include all the weight that you need to redistribute. To me this is a value used to determine the size bars that you need for the WDH. It does not change the amount of weight that the Hitch is certified to carry as that weight is calculated by adding the weight added to the hitch receiver such as tongue weight + WDH weight.
I have never seen any warning by the OEM or hitch receiver manf. to the contrary that adds any caution to add the weight in the bed behind the rear wheels. Once again I think the confusion comes perhaps from the sizing of the WDH bars.
