Also, the gov't can't really tax it (pot), because any one can grow it. That's really the reason for it to remain illegal, they can't regulate the sale of it.
But people still pay a great deal of money for it (obviously, from the topic of this thread) , so it can be taxed.
People pay a great deal of money for "Indian cigerettes" as well, they are tax free. (Yes, because they're made on reservations, but my point is it's do-able.) If it's not illegal there would be more people growing it, aswell as consumming it. Sure, some would be large companys, but many others will as well. It would also be un-constituional to limit the growth to spacific companys insead of every one.