The BBC instance is trying to fulfill a different role than mstdn.ca fulfills. The social.bbc site (and presumably a similar CBC effort also) is not open to general membership, strictly BBC employees and content. So conceivably a CBC journalist could have a social.cbc.ca account for their work and a mstdn.ca account for their personal life.

@Knightfall @grte

On a cost level, yes it would make sense to leverage mstdn.ca but issue is that CBC wouldn't have the complete control & the instant brand recognition via hosting their own instance on their domain name.

It's also much better for the brand to have an account on social.cbc.ca. Something like cbcnews@social.cbc.ca looks better on a branding perspective than cbcnews@mstdn.ca. It's no different than how organizations use their own domain for emails.