Per impression payments are more vulnerable to fraud...

Hide the ad unit in some hidden background layer and you're still gonna get paid for impressions even if no human eyeballs see it...

And you'll get away with it too, because as long as you have millions of web pages and each one only gets a little traffic, there won't be enough statistical power to see that they are underperforming vs just unlucky that none of the first 85 impressions led to a click.

This is already rampant in mobile games.