Useful Analogies

Now that we’re in the last few months of the Era of Pop-Ups on the web, I’m kind of curious what the annoying ad format means. I think pop-ups are distinct from pop-unders; The ones appearing behind your browser seem more like a slime trail left by aggressive advertisers, whereas those first X10 ads always seemed somewhat more invasive.
It seems to me that pop-ups should be analogous to the tear-out cards in magazines, a form of promotion that calls attention to itself and is intrusive but somehow doesn’t seem to be against the "rules" of the medium. Yet everyone I speak to thinks pop-ups are damn near offensive and has no problem plucking all the tear-outs from a magazine or simply ignoring them while reading. Even accounting for the learning curve that some users will have just figuring out how to close these windows, there’s a visceral reaction that I find fascinating.
Why is our experience of pop-ups on the web qualitatively different than other forms of intrusive advertising?