Because WSJ (and other sites) filter access based on the referer header. They've made a business decision to allow access to people following links from Facebook, likely in the hopes that the readers will find the article useful and be more inclined to subscribe. They also benefit from the views such links generate.

Facebook de-ranks publishers that have paywalls. Since these publishers want Facebook traffic, they don't show paywalls for people that come from Facebook. One way to implement this is by looking at the referer.