Why do some proxy servers, offer their services for free?

I find on the internet many websites that offer lists of "Free proxy servers" situated all over the world, and I've used some of them to watch geography restricted websites, like the  German TV channel ZDF from outside Germany and they seem to work fine.

But I can't explain why would they do a thing like that? Why would anybody open a server in Germany just to hide some random's guy true location. Servers have costs and I'm not paying him anything, do they earn money other ways? Like selling people's information or inserting ads etc?