Makes perfect sense to me. The teams running in Nextel will have literally hundereds of cars to get rid of, instead of pumping them into the local short-track scene they are being put to good use with different body shells on them. They will remain the same cars, but different bodywork. Making the Bush series a very cheapish division to run in, until the cars run out that is.
There isn't much of a difference now between the car makes. the bodies are nothing like their real life counterparts. Running a different skin is not going to change that =P They all produce about the same hp and torque. There are subtle differences in the areo, but cup cars now are nothing like they are in their every day counterparts. It's all about marketing now more than anything. Sure it says Dodge, or Chevy on it, but you could take an RX7, put a few pieces of bodykit on it and call it a Viper. Anyways this is going OT =P It's a good move to bring the cup cars to bush, but the problem will now be making them slower than the COT cars, least IMO they should be. Chances are they would put restrictors on them to limit them to something like 600 HP or so.