Total conjecture, but here goes...
I've had friends host their videos on Thrasher either for free, or Thrasher threw some change their way. The brand is not in a position to charge to host mostly meaningless content that will likely get lost to the ages. They need the constant stream of content as much as people who host their videos on Thrasher need the "exposure," or whatever. It's a symbiotic relationship.
I'd imagine for big releases and guaranteed clicks, like a Zion Wright part, there's a deal made, but I bet most of their revenue likely comes from the pre-roll ads we all have to suffer through.