...you can book award travel with United miles from LAX to YUL, but not the other way around?!

Try this: On the United site, search for LAX-YUL round trip, cattle, for all of July, and filter for nonstop only. United has Air Canada codeshare offerings every day for convenient nonstop flights. But switch over to miles (or run an award travel search, same thing) -- and you're forced to book your return with a connection. If you'll force the site enough, it will admit that there are "no options" for YUL>LAX, in miles, nonstop, anytime in July. Switch back to $, and they're all there.

It's as if AC told UA "you can sell award travel on our shared LAX>YUL flights, but forbidden on the return leg!" Is this normal??