The thing is, anybody can build a front end to the Steem blockchain and decorate it however they want, including with ads. It's really no different than if you built a web front end to usenet newsgroups (which was frequently done in the more ancient days of the internet).

You still have ownership of your content but what that means is necessarily limited by the nature of a blockchain. For instance, once something is put on the chain, it can never truly be deleted, ownership or not. You can of course delete a post but all versions are still part of the blockchain and can be accessed.

As far as the use of the canonical tag, what if every front end started using it? Seems like it would negate its value. I'm not even sure the canonical tag even really applies since it is that blockchain that is the true authority. But again, any front-end designer can do whatever they want. There's really no point complaining about it. In theory, a tribe could implement some kind of encryption as has been mentioned and that would probably take care of it. But then you are limiting the audience of your post which may not be desirable. Kind of defeats the point really...