Its not just inventory issues. US is more often than not pricing the same flights on UA metal for up to 5 times the amount that UA is selling them for. Its just another symptom of the mis-pricing sickness that US has in too many routes.
Expect the code-share to unravel further. UA is going to withdraw more codeshared flights as the merger with HP unfolds. Quite simply there reallly is little benefit to codeshare tickets anymore. Barring any extraordinary price differences, I fly US on US metal and UA on UA metal. I may accrue US on UA and vice versa but that has nothing to do with codehared tickets.