You're not getting a free lunch. The sites are riddled with ads. Each impression may not net much revenue, but add up all the views by "greedy" peeps coming to grab "free" microbitcoins...
![Grin](https://bitcointalk.org/Smileys/default/grin.gif)
This guy's got it right. They use any form of advertising that brings in revenue.
(Banners, text-link, popups, captchas, etc...)
These make them much more than the few fractions of pennies they give out.