I'm not surprised airlines do this. I don't think it is just Air Canada. In a way it makes sense but it does make me think they are a bit greedy.

If someone buys a ticket and does not show up for a flight then usually they don't get a refund/credit (or a crappy one: I'm looking at you United).

But now the airline has an empty seat. The best thing to do is fill it. But you can't fill it the last moment so you need to overbook. But if you overbook too much then you need to start bumping people off. So the airline gets to keep all the revenue from the overbooking as well as the fees for the refund which can be substantial. It's a bit like double dipping.