Usually pay-per-click is done not literally as a pay-per-click but pay per hundreds or thousands of clicks. That and each click is only a few cents on the dollar. That way it takes more legitimate clicks to make more money. Not to mention, many sites just push their users to visit their sponsors. So many times it's considered "spam".
Another option is to not credit the referring website unless that visitor goes to more than one page. This helps drive visitors into your site rather than keeping them on whatever page was linked to. There are ways to check this (sessions being easiest, cookies being used to help combat multiple clicks from same IP).
That's my basic understanding of it. It's not literally a 1:1 ratio of clicks to $$, but more of a 100:1 or 1000:1 ratio.
