They get a really high CPM for the ads they place, so it's not all about the pageviews. If you wrote a bunch of linkbait or put a few slideshows up there, they might not have the same audience, and therefore wouldn't get the same CPMs they're getting now.

So not knowing the pageview count might actually help in the case of PandoDaily.

Presumably they still reward good performance though, so writers are still incentivized to attempt to get good pageviews, even if they cannot know the exact numbers themselves (and they can probably get reasonable estimates by watching mentions in social media)