To use the possibility of allowing standby as a selling point would be a valid explanation if it was consistent accross the AC route network. Not only is standby disallowed only on domestic flights, it's on selected domestic flights with competitors that offer high frequency.
YVR-YYC Fare basis: N7WOW - same day standby allowed
- WestJet competes on this route, offers relatively high frequency and allows for same day standby (always)
YVR-YHZ Fare basis: LLECONO1 - same day standby not allowed
- JetsGo offers flights in this market, but only one a day, so standby on JetsGo is not possible.
YVR-YYG Fare basis: LLECONO1 - same day standby not allowed
- No competition
Same day standby is also allowed on transborder flights (with competition from US carriers) as well as other international flights.
It looks to me that AC is only to nickel and diming where it can (where there is no/weak competition).