People who fly CO in F. (And I'm not talking about myself, because my "free" upgrade percentage on CO is abysmal, due to all of the paid F fares.

) US prices their F product much higher than CO, and you get a lot less. As a result, when customers have a choice and are prepared to pay for F, they fly CO.
I tried to did sample fare comparison PHL-IAH-PHL non-stops on Saturday, July 24, returning Mondday, July 26 to prove my point. My findings were as follows:
1) USAirways.com is showing F sold out on all nonstop routes both days on their website, which I find very hard to believe -- especially considering that July 24 is a Saturday.
2) If you search for Preferred Upgrade Fare (i.e. instantly upgradeable upgrade for elites), the R/T fare is $1,364, including taxes and fees. You get 100% miles & elite qualifying miles. If you are not Preferred, you cannot even buy F, although I suppose if you call and are willing to pay $25 on top of the F fare, an agent will book you in F.
3) If you search on CO's site, the lowest F fare is $929, including taxes and fees. If you are a Platinum on CO, you can book an M fare for $585.90, including taxes and fees, and confirm F instantly. Silvers and Golds can buy a B fare for $809.90, including taxes and fees, and confirm F instantly. AND B fares earn 150% RDM's and EQM's. And get this: If you are not an elite with CO, you can actually
buy F,
because their website works!!!
I agree wholeheartedly that it's bad business to "give away" your product, but the way US has approached their revenue management has created a scenario where nearly everybody sitting in F is an upgrader on a cheap ticket. They cannot even sell F to those who want to buy it, because the website shows it as sold out. But of course, it's the customers' fault, because all we really care about it free upgrades.