(function() {  
    // local scope, can't override
  })();

My point is that I agree with grandparent, that they just allow ads to be shown, and it's not technically impossible to reload ads, or prevent page contents view, if no ads visible.

The plugin can absolutely mock the ad element(s) and make them seem to be visible.

only because sites generally don't do so; if necessary, extensions could get much more aggressive with website scripts, current user script limitations aside.