There's nothing "noble" about spamming search engines with full text results then putting up a login-wall for the same content you arrived on the page for.
Tricking random users looking for an answer that's supposedly there into registering so they can see the 1 answer is not increasing engagement, just signup counts.
How is it that Google doesn't penalize them for this?
I thought it's considered a big no-no to show people something different than what you show the crawler.
We don't care about signup counts, only about real engagement. Most people have no problem with registering, and the engagement from logged in users is so much higher that it more than offsets the small minority of people who don't want to register.
Tricking random users looking for an answer that's supposedly there into registering so they can see the 1 answer is not increasing engagement, just signup counts.