That's only possible because they have very sensibly decided to avoid that arms race. Why? Because ultimately, the ad is playing on my monitor, linked to my com

That's only possible because they have very sensibly decided to avoid that arms race. Why? Because ultimately, the ad is playing on my monitor, linked to my computer, running my browser on my OS. All that stuff is under my control. If I really don't want to see ads, and yet want them to think I'm seeing ads, then I can. I have seen people put stickers on their monitors over where the ads are... How are they going to detect that? I'm sure they feel it's much better to be able to tell when people are blocking ads (and therefore not charge for impressions), than it would be to try and force me to watch ads, when I've already demonstrated that I'm willing to do what it takes to block them.