If there is a deal that can be made to greatly benefit a CPU-controlled team, and the Trade Council approves it, trading with open teams will be allowed.
However, if the deal you propose is made "just for kicks," or, in the eyes of the council, is not a trade that should be made by the CPU team, it will be disallowed. Trading with open teams will only be allowed if it is clearly rational from the perspective of the open team; i.e., a CPU team has 7 ML-ready starters and you want to trade for one of them.