They want to track you to count articles you have read:

> Visitors are allowed 3 free articles per month (without a subscription), and private browsing prevents us from counting how many stories you've read. We hope you understand, and consider subscribing for unlimited online access.

However you can easily defeat this by:

A) Using a different browser session and just clearing it once you read 3 (they don't appear to be using advanced tracking).

B) The article is still actually sent, just remove the div in the body element with class "incognito-wall", then click on the body element and remove the "overflow: hidden" style.