Why Your Ad Blocker Doesn’t Block Those ‘Please Turn Off Your Ad Blocker’ Popups

Isn't it obvious why? The website has detected the ad blocking and is crippling its functionality somehow. If the accompanying warning is suppressed, then the user doesn't know.

The fix for it would be to do ad blocking which is not detectable. That is impossible, since ads are something that the browser basically pulls from the site (or its associated advertising domains); you can't hide the fact that you're neglecting to access ads (unless you wastefully access them and then just hide them from view on the client side, which defeats the bandwidth-saving aspect).