I've brought up this issue before and the consensus was the "antitrust factor."

Don't forget that CO and NW have valid routings for published fares. I'm making this up, but say NW EWR-LAX might include routings via MEM, DTW, MSP + CLE and IAH. Then, assuming that the fare class is available, it would be the same price going on NW metal via DTW as a NW code-share on CO metal.

The situation that I came across was quite a bit different, it involved HP. Long story, but it involved fare rules and minimum stay requirements.