Sounds pretty formulaic. I flew a UA codeshare LAX-YYZ last summer and neither airline treated you like a passenger of their own.

I had to get to Pearson early (really early) and tried to stand by on a UA flight through ORD. Impossible, the UA CSR said, since I was ticketed on an AC flight.

I tried standing by on an earlier AC flight and was told that AC had just changed their stand-by policy and I would have to pay something like $45 CDN...aaargh.