Did anyone ever figure this out? It looks to me, based on the vehicle bulletin mentioned above and the labelling on the cab headliner, that the head side airbags deploy towards the cab. My guess is that the blocks serve to pad the airbag deployment and your head in case of roll-over or roof deformation of some sort. I don't think that the blocks break away as some have suggested, it seems to me that the bags deploy under the cab headliner on the side. The headliner is pretty flexible, whereas the blocks are not. Also, the blocks are held in place by almost impossible to remove plastic clips. I finally did manage to remove them, though.
Also, the vehicle bulletin doesn't have an accurate picture of the blocks in my cargo van. I wonder if there's a separate block on passenger vans that they're talking about.
I think I may do what others have done and upholster the blocks. I've been wondering about cutting them to allow my finished roof with headliner go behind it. Has anyone else done this?