This is not only a Ford thing, the other big 2 do it too.
My F350 is a 2005. Still an 11,000 GVWR with a 7,000# Rear GAWR.
This is what come on mine in 2005. Hey at least yours is 850# in weight carrying mode. But this receiver did work very well up to 1,250 in WD mode. Just I needed more
I needed to upgrade it to this to handle the 1,600# loaded TW of my camper. A Reese Towbeast 2.5" receiver
A number of years ago the big 3 got together at a conference and "again" agreed to create receiver ratings that lined up with a 10% loaded tongue weight with the full tow ratings. There thought process I'm guessing is that the rule of thumb that 10% is needed for a stable trailer. The issue is, on a TT, the loaded TW can far exceed the 10% and you run out of WD capacity on the receiver before you run out of pull rating.
Is it lame? it is the way it is unless we can convince the big 3 at their next conference we need more WC & WD capacity. GM "use" to give you more loaded TW in WD mode then the truck could handle even. Well that at least gave you a better receiver but if the truck cannot handle it, someone is gonna think they can do it and try. Not many people read and understand all the fine print.
Lets say this, at least be glad on the new 3/4 and 1 ton trucks we now have a 2.5" receiver as standard. They had to with all the towing torque wars going on. The truck can pull more so it has to be able to hold up more by the receiver. It use to be worse....
And I do agree, even with a 600# loaded TW on a trailer, I would use WD. The truck just rides better.
Hope this helps
John