Note that physicsforums.com did not choose that ad... Google did.
Google pays sites for the right to display ads of their [Google's] choosing, based on keywords that appear on the site. In turn, advertisers pay Google in order to have their sites be advertised on other sites that contain specified keywords.
Click on the "Ads by Google" link for information. There's also a feedback form. If enough people complain about crackpot ads on this site, maybe Google will try to adjust their algorithms accordingly.