Not true - You can, and should be able to get seat assingments on Air Canada operated flights. It is possible that there are no assigned seats left to give out on your particular flight, a Web site snafu, seats got transferred to airport control for some reason (it happens), aircraft change coming, etc. If seats are available, a call to AC shoudl rectify this, although wait until the weather calms down a bit everywhere, so you don't spend hours on the phone. When are you traveling - if its in the next few days, ops are a mess and flights are booked solid, which may lead to your issue - in which case, you should just be happy if you are able to get out on your flights.

To be fair, a lot of codeshare flights don't allow seat assignments automatically - At least on UA flights booked through United.com, operated by AC or US you can also not get seat assignments. Air Canada res. agents might be able to request a seat assignment for you on the codeshare flight as well, if not, call the operating airline, and they should be able to help you.