I have been noticing this since quite some time now. Google AdSense has issues with some keywords if they appear in a web page URI.

Porn is one such keyword. No paying ads appear on web pages which have PORN in its link address.

Check this page for example!

This happens even if the page has nothing controversial. A story about China’s crackdown on PORN with this keyword in the URL would not show any Google AdSense ads.

I have seen this happening on major sites like Mashable and The Huffington Post.