A rather suspect SEO paywall trick is to allow web spiders from search engines to see all content but lock the content behind a paywall for all other visitors.
As this unfairly advantages dishonest websites over honest websites in search results, why doesn't Google either:
- Mention that a website is behind a paywall; or
- Demote the website's search ranking by some sizable margin?
Given the skill of Google engineers, it would be trivial for Google to act as a normal visitor and check for selective paywalls.