Again, please explain how any part of the system knows whether it's level or parallel to any other thing. The bars are bent. It's geometrically impossible for them to be parallel to a straight frame rail.
Regarding your second point, look at the bar on BOTH sides of the cam, and both sides of the tongue. When making a turn, one cam moves forward, the other moves rearward. Yes, you are correct that the cam that moves to the rear is ramping up less of an incline. However, on the other side of the tongue, the bar is moving forward up a much steeper slope. Admittedly I haven't crunched the numbers, but I'd wager that the difference in correcting force between bars whose 'hook' on the contact point is more or less level compared to the force from bars with the angle seen in the OP's photo is pretty minimal.
I recall having a similar discussion here a couple months back... Someone found ONE manual somewhere which made a vague reference to the bars being parallel to the frame. (I think it was a round-bar from Blue-Ox, maybe?) None of the rest did, if my memory serves....