It's because people who play the game will buy things to get ahead for 99 cents here, 99 cents there. It might only be five or six bucks a month per person, but added up over the millions playing, it's quite a haul.

Here's an article that talks a bit about the whole economics of game development, but not quite what I'm getting at here:

http://www.socialtimes.com/2010/03/the-economics-of-facebook-games/