Because you're in private mode? You can still read the text of the article, as it's briefly displayed before being obscured. While the full text is visible there is enough time to do an alt-A (select all) clt-C (copy) key sequence, then at your leisure paste into your favourite text editor and read.
There is a 5/month limit stored in your browser as a cookie and checked by some javascript. If you haven't read 5 articles yet in that browser, or you have scripts or cookies disabled, that'd be why.
