How would you enforce that? Who can be trusted with the ability to classify things as advertising and in so doing silence their critics?

The solution is explicit trust. Not fingerprints for users, but digital signatures for content. If it's not signed by somebody the user trusts, don't render it.

The user is the only one who can be approach that task without a conflict of interest. If somebody wants the user to run code that phones home about user behavior, make them gain the user's trust.