There's a lot of people who post these sites that claim to pay you per download or something along those lines. Why would they pay you per download? They have to pay for the bandwidth and now they have to pay you for making them pay for bandwidth? Maybe I'm confused.
I forget which site it is but it's a popular download site and what they do is they have ads that look like the download link. It's very easy to click the wrong spot. I know I've done it before. If you have enough traffic you can just sell ad space.
Along with the ads, they get better traffic and more importantly more premium accounts which makes their pockets heavy !! And yes some sites like sharecash also relies on CPA offers like surveys which needs to be completed to get 30 cents or whatever per download.