Air Canada, along with other Canadian airlines, hasn’t been refunding tickets when they cancel flights. The Canadian government has backed them, though for flights to or from the U.S. it’s a clear violation of U.S. law.
Telling customers they can only have a voucher, to use later, is effectively an interest-free loan (they hold onto the money until they earn it later) and a risky one (risk that the customer will have an opportunity to fly, and that the airline will still be in business).