Why do hitch adapters (riser or lower-er) reduce weight capacity?

d r

New member
I have a class III hitch rated at 600lbs tongue weight. I want to put a 6" riser on it:
http://www.etrailer.com/Tow-Bars/Roadmaster/RM-076.html
I was told that using one of these will reduce my tongue weight. Why?
 
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