i wonder if they do this so their content can still get indexed by google
Yes, you must show a Google user the same page that the Googlebot sees or you risk being de-indexed altogether (they've done it, with some pretty major sites, to send a message). That's why if the click doesn't come from Google it's paywalled, but if it is from Google it isn't paywalled. The practice of showing a different page to users than to the Googlebot is called cloaking and Google hates it.
Google's business strategy you see is to put their ads on other people's content.