While I mention this I have a question: why should web sites only make money if people click through? After all magazines, radio and TV ads don't pay the publisher/network if people phone them up.

A very good question. The reason is simple: advertisers demand click-thrus because they can. It's the same with metrics -- you can't say for sure how many people actually see a full-page four-colour ad in a magazine, but on the net, you can measure a great many things and just because you can the advertisers feel you should. Internet advertising is so dramatically misunderstood by traditional advertisers that there is a titanic, ongoing struggle that most people aren't aware of that could very well change the way we use the web in the future.

But that's another story. :)