So, why keep returning search results that the end user can't use without registration or purchase? It's essentially "page cloaking" when the rendered page doesn't match google sees.
To me, if you want a paywall, that should come with the consequence that your site isn't included in search results for the general public.
Edit: It's also getting irritating here on HN. I might have a subscription or login to one or two sites, but HN regularly shares stuff from Medium, WSJ, NYT, Wired, and so on. I have to imagine that most people following these posted stories hit the reg/pay-wall.