There was a post here a while back (but I don't have a link) where someone said they'd contacted US about this and were told they won't sell the UA codeshares until they'd sold sufficient numbers of their own flights (or maybe until their own flights were full). I may not like it, but it makes business sense IF you can make more flying your own plane than sharing the revenue with the other airline. From some previous disucssions, they may make more with the codeshare revenue than flying their own planes...