Some might say that Pinterest's behavior is covered under the "Spam and Malware" section on that page, but clearly Google does not agree with that interpretation of Google's rules or else the search results would have been removed by now.
There are other sites requiring logins that appear in Google results, e.g. LinkedIn, and Google clearly doesn't see that as a rule violation either. If that was really a rule, Google would need to block a lot of major news sites as well, e.g. NY Times, WSJ, FT, Economist, etc. I'll believe Google's interpretation of their own rules, rather than someone's interpretation on Reddit.
