The problem with open sourcing their IP is that Fusion Garage could then turn around and start manufacturing the devices, cutting Arrington completely out of the loop. They would just have to be careful that the separate Arrington IP doesn't taint their existing proprietary parts of the Crunchpad.
Also how would he ever let Fusion Garage "jointly own" part of the IP? That sounds like a terrible idea. The only way that happens is if his "team" couldn't actually make the thing work (which is what I'm reading between the lines). So he's over a barrel because he promised WAY too much and couldn't actually make it happen.