Sorry for me delay in answering this. It does in fact seem they did design it so that if you had Cam Plus Unlimited all your cams would be moved over to that. They thought this was a helpful move. There are discussions now on changing that back as I have brought different use cases on why you would not want all your cameras on CPU.
Some of these reasons are ones I have seen here where maybe the user does not want one recording. Another reason I have run into is users who beta test would remove one to also test the experience outside of Cam Plus Unlimited and they have lost the ability to test those things.
As it currently sits if you have CPU all your cams get transferred to CPU and there is no way to switch one off, but this may change.