Because google does not like to index pay-walled articles. They are either not indexed at all, or given a low score. This makes complete sense because Google wants their users to be happy and users are usually not very happy when they hit a pay-wall.

Online publications, on the other hand, really want their paywalled articles to be indexed by Google. You may remember Murdoch did a lot of complaining about this a couple of years ago. So now publications do a little trick where they make the whole article available to people coming from Google but offer a paywall to anyone else. This means that you can usually access the entire article if you are redirected from google.