There are literally scores of different reasons why someone might choose to distribute Free software: that's why there are scores of different F/OSS licenses. My favorite reason for going Free is from Linus Torvalds on why he chose and sticks with GPLv2:
Me, I just don't care about proprietary software. It's not "evil" or "immoral," it just doesn't matter. I think that Open Source can do better, and I'm willing to put my money where my mouth is by working on Open Source, but it's not a crusade -- it's just a superior way of working together and generating code.
It's superior because it's a lot more fun and because it makes cooperation much easier (no silly NDA's or artificial barriers to innovation like in a proprietary setting), and I think Open Source is the right thing to do the same way I believe science is better than alchemy. Like science, Open Source allows people to build on a solid base of previous knowledge, without some silly hiding.
But I don't think you need to think that alchemy is "evil." It's just pointless because you can obviously never do as well in a closed environment as you can with open scientific methods.
This goes to Eric S. Raymond's Linus's Law:
Given a large enough beta-tester and co-developer base, almost every problem will be characterized quickly and the fix obvious to someone.
Or, less formally, "Given enough eyeballs, all bugs are shallow."
