The consideration with open source licenses is often authorship credit / promotional. It's not terribly different than a band giving away MP3's of their music:

The consideration with open source licenses is often authorship credit / promotional. It's not terribly different than a band giving away MP3's of their music: maybe they want to share their music with the world, maybe they want you to buy a ticket to their concert, but they are getting something out of it and are free to give away something if they have the rights to do so.