How would you ensure that the news service doesn't accept side-channel donations for the "right" news? How would you ensure that the news service doesn't keep telling the subscribers what they want to hear?
The news service could have an independent accountant affirm that all revenue comes from subscribers. As for bias, this is not something I'm suggesting would be prevented. I'm only suggesting it would be a news organization incentivized to serve the readership and not advertisers.