I'm not an artist/designer, nor have I ever done spec work as a developer, but it seems to me a lot easier to decide to help a not-for-profit organization (Mozilla) aimed at general societal benefit for $0 than to do so for a for-profit company.
Of course. But if a for-profit company came to me, I would consider doing the same for them. I may take it. I may not. But my point is that it's not evil. It's a case-by-case thing and that if a designer wants to work for free, why are others getting up in arms about it. It DOESN'T affect them.