A light bulb just went off in my head:
Look at this photo from the Reese web site:


Note how the snap-up extends the entire height of the frame. That makes a lot of sense to me. the bottom of the snap-up is at a strong point on the frame, not one of the weakest.
Now this drawing of how these work on a 6" tube frame:

Is it reasonable to speculate that these snap-up brackets are simply not designed for 6" frames and that the original design took into account preventing stress on the middle of the frame and that somehow, with these larger frames, Reese may have simply dropped the ball?
[edit]
OK I found some pics (including my install instructions) with the snap up not reaching the bottom of the frame. I still don't understand how a huge portion of the load on the snap-up does not end up trying to fold the frame in though (without the bolt at the top).
[/edit]