I'm not sure I understand why they exempt Google refers.

Can someone provide an explanation of why it's beneficial for them to exempt Google but not other links?

easy. They want Google to index that content and they want to show up in the search index. At one point I think some sites tried showing one thing to Google-bot and a pay-wall to the rest of the Net. But Google realised that was a terrible experience for users -- I've googled somethign and now when I go to the page that google recommends to me - I can't see what I'm looking for.

So Google effectively laid down the law -- If you want to show up in Search Results -- then users who have found you via Search need to be able to read what you are showing..