Tim O'Reilly posts his answer to the question "is O'Reilly a free software business?" that was made on the free software business mailing list. He also points to an answer made by Simon Cozens that he particularly likes:
For programmers, writing software is fun. It's actually enjoyable to produce a free version of something you "can't" have because it's proprietary. Why do people write free software? Because it's either necessary to have a free version, (whether pragmatically - we need a free version of this software because there isn't a commercial one we can actually use - or dogmatically - because we believe that everything should be free) or it's fun to do.
Documentation is different. For most programmers, writing documentation isn't fun. So the question is, is it necessary to have a free version? If there's a reasonably-priced book available in a shop, it's probably not pragmatically necessary. That only leaves dogmatic necessity. And that's pretty weak - the only people who are really interested in dogmatic necessity are those who spend more time playing politicians than they would writing documentation anyway.
