The movie "Fight Club" has an interesting line just after the narrator moves away from his "normal life" and into the derelict house:

    By the end of the first month, I didn't miss TV.

I suspect most people don't understand how subtly addictive TV can be, as it takes at lest that month to break the habit. While I'm not sure if it's part of the same effect, there seems to be something similar for advertising: you don't realize the huge cognitive load it puts on you until you've been away from it for a significant amount of time. Running a firewall isn't free, yet the costs associated with recognizing that something is an ad which should be ignored are often assumed to be trivial.