The dice run atop everything on the board. Tokens aren't 3D objects. They lack a physics shell to factor collision. The only things that have physics applied to them are the dice models, a "floor" or "table" physics shape that the dice roll atop of, and the edges of the tabletop portion of Roll20 are treated as physics walls. Note: I'm not talking about the edges of the playable space, but the entirety of this portion of this screen that the tabletop inhabits. If you zoom out to 10% and roll some 3D dice, you'll notice they will roll completely outside of the actual playable space.