They have to be very careful -- Google's guidelines are very strict about content that they index. If you display different content to a user than the content that Google has indexed, that will cause Google to drop the page from their index, and may affect the overall quality rating for your site in Google's index.

Nobody really wants to cross them here; if you want your content indexed and displayed in search results, you have to show the full content to users inbound from search result pages.

Not exactly. Google actually allows some sort of "google bot" login and see as well as this : https://support.google.com/news/publisher/answer/40543

True, but they could just show google the same thing they show users who are not logged in. They just choose not to.