[DougWebb]

"Trading schemes add a lot of unnecessary complexity and potential for abuse..."

I believe that's exactly why they are "preferred" by our political system. Whenever the political body starts to consider laws about pollution, the lobbyists for the polluters go into high-alert to influence the outcome. They don't want a straight tax, because that'll become a simple cost which is difficult to avoid. Instead they prefer a complex scheme that can be sold as if it has the same impact as the straight tax, but in reality it will provide ways to game the system to either avoid the costs or even make a profit for the polluting companies.