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 serve the same content to Google that we serve to users. We have no interest in deceiving them.
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.
More detail at my comment here: https://news.ycombinator.com/item?id=7808698
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.