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..