It's also a mechanism of price discrimination. Using some third party site is an inconvenience, so anyone who can afford it or wants to support the site is going to subscribe. Having a slightly inconvenient but available path to reading the articles for free is a form of advertising (people who might subscribe can sample articles first), and most of the people who will always use the workaround were never going to subscribe anyway. Also, they may want the story to actually be archived -- who else is going to maintain a copy of it for the historical record when the company goes bust?

On net it probably gets them more subscribers to let it happen. But then their traffic declines anyway for other and industry-wide reasons and some shortsighted tightwad concludes that allowing it is costing them rather than making them money.