OP specified they used ticket number, not PNR. I've not seen a case where UA and OAL PNRs on the same itinerary match - the only way this usually happens are on PNRs where the airlines involved all use Amadeus, which IIRC, always uses the same PNR. For example, a PNR that involves both LH and TG. Some airlines allow you to access using just ticket number and name.

As for CA, they aren't alone - Flip those letters around, and you get AC, which also doesn't let you select a seat online if you have booked it anywhere other then their own website. You can't even do it when booking an Aeroplan award. Not sure since they've done the switch to a new system, but NH only allowed you to login on flights booked with an NH code, meaning if you booked on a UA codeshare, you couldn't access seat selection (or even bring up a booking) on their site).

I've never flown CA, but probably not that big a deal to call to get the seat assigned.