That is their policy, but they don't appear to punish people who violate the "spirit" of it and many of these sites don't literally violate the rule.

In the case of pinterest (and quora), for example, the whole page you are looking for is there, it just gets obscured by an obnoxious overlay that makes it impossible for you to see/interact with it all from a browser unless you register.

I'm sure what the googlebot gets at the html/css/js level is the same thing the user gets. The difference being the bot doesn't care about the semi-transparent blocking overlay because the bot is just parsing text/images/links individually and not trying to interact with the page the way a human does.

Not anymore apparently, at least for the big players. I believe it is still the stated policy and you won't get away with it if you are small.

Pretty sure it is.