It’s a form price differentiation, really. Also, I’m guessing this is probably the reason why many are not aware of the paywall and link to these articles.
In this case alternative and equally good sources without paywall exist so it doesn’t make much sense to link to the article with paywall (but that might have been a mistake). However, I do not understand objections to paywalls in principle.
Sure, some people will have subscriptions, others will have found the article unlocked in one form or another, but still I think it's pretty poor form to share something closed source, if you will.
Here, look at this: http://www.checkupdown.com/accounts/grpb/B1394343/