Actually, the website and kiosk won't let you buy a ticket there because US does not have a codeshare agreement with CO - just like you can't by a ticket to some obscure Canadian city since US has no codeshare agreement with AC either. Aside from the standard ability to earn/redeem miles with any *A carrier, the only provision that US and CO currently have is reciprocal club access although Kirby has hinted at codesharing with CO in the future.
While I want US to give off a professional and polished image, the bottom line is that it is impossible for someone to be impacted by this oversight. It's threads like that make it appear that this forum is just simply a bunch of trivial and incessant complaining and detract from the real concerns/problems that actually are brought up here.