Subbing to this as I have always been unclear on it myself.
My personal opinion is that the bars are semi-load bearing. That is, they may make limited contact with the ground, basically just barely touching on impact.
If a horse was in the wild and had their hooves worn down by natural forces (stones, bedrock...) it seems to me that the bars would have to make full contact to wear if they are as hard as the walls. However, they are definitely not as durable as the walls, so to me that implies that they would shed kind of like the sole.
I like Mike's conception. Best one I've heard so far ever.