There are several sides to this, ex:
Is it ethical to advertise something that users (or indexers) expect to be freely accessible, and then to pull a bait-and-switch when they arrive?
Is it ethical for ads networks to spy on people, fingerprint their machines, and create dossers about them?
True, several sides, 1. It's not bait and switch, you have a preview of the text and if you like continue, its upto platforms which publishes the link to mark it as paid or free - or by crowdsourcing. 2. Just because a model isn't perfect that doesn't justify a illegal or unethical action. If you don't like it, don't consume.
I agree with you on advertisements, I don't like an ad ridden internet either, but I have come around to thinking, the alternative and the incentive to ad free internet is an easier payment access