My reservation is viewable on US's website. I tried to choose seats there, but when I got the seat map, every available seat was a "choice" seat, available only to Dividend Preferred (which I am not). So I couldn't choose anything. I don't know when these seats become available, but I hope they open up at T-24. There are some fine seats open, apparently--I just can't grab them.
None of this answers my original question, which is why I could buy this seat via Orbitz but not from US itself?