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...
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.