Brent,
I'm sorry, I don't know the specific reason/s behind the recommendation. Whatever I say would be pure wild-a$$ speculation.
I have not spent much time studying weight-distribution platforms vs dead-weight platforms. It may be in the platform or it may be in the wheelbase of the...
The 80 series is rated to dead-weight tow 5000 lbs trailer/500 lbs tongue with a properly fitted platform and appropiate trailer brakes.
Toyota does not endorse the use of a weight-distributing hitch on the 80 series.
D-