Good point, but they would probably be "good enough" for a lot of people. The friend that I bought one for said he was not worried about it.
If you are only using round overs and chamfers, it would probably be fine. I want them for beading bits, cope and stick and other things that require more precision. A plate that is not flat will change the angle that stock is presented to the cope bit and pull the resulting joint out of square.
I may call Grizzly though. If they want me to ship them back, it probably will not be worth the effort.