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
My concern is less the page itself showing blurb-with-divider, but what happened a second before when someone clicked the link.
Is there a standard way that these sites are up-front about "Most people won't be able to see what I am choosing to reveal to you" which is communicated to search-engines, and the search-engines are at fault for the bait-and-switch?
> Is it ethical to advertise something that users (or indexers) expect to be freely accessible, and then [...]
So if you expect something to be free therefore it must be?