I think this doesn't concern stock photography. Looks like a social network donation system where you can donate to someone offering something for free - like freeware fonts, flickr photos etc. If someone uses the stuff you offer for free they send something back.
And the person pays to be a member and click the buttons. So we can assume that the flattr site takes a percentage for operating expenses and distributes a percentage for the clicks, divided by the monthly subscription fee. I didn't see if the people who provided content were working for nothing, or will have to pay to get those Flattr clicks.
Hypothetical, you pay $10 for the month to get free content and makes clicks. You click only one site, because you want that person to get the whole pie. They get $2 and Flattr gets $8. Simple!
Make the people who run the site rich, off selling subscriptions for free material.

Someone was thinking when they thought this one up. All feel good messages about sharing and liking. Just can't wait to see what they are pocketing for providing this service and love.