The point isn't to make the user follow the ad -- the point is to force the user to parse the ad as a step towards something the user wants. The theory is that

The point isn't to make the user follow the ad -- the point is to force the user to parse the ad as a step towards something the user wants. The theory is that this gets a message into the user's brain in a way that traditional advertisements do not. While it looks like a captcha and appears in similar contexts, calling it a captcha is misleading: It fulfills its primary function even if it lets all the bots through.