I thought Google users were only privileged because news sites want the full text of the page to show up in searches, and Google punishes pages that show different content to the web crawler than they show to searchers.

and the reason is: if a user bounces off a SEO result, the ranking is degraded, and nobody wants that, so Google users can skip the paywalls. I don't think that's a good reason, or even a good way to protect content.