Morality of Using Open Source Software

I’ve been giving some thought to whether it is right to use Open Source Software. On the one hand, by using OSS I contribute to an unjust system, where undeserving free-riders can benefit from the work of unrewarded developers. On the other hand, these unrewarded developers — the altruists who create valuable software with no intention of collecting payment for their work — I have to assume are intelligent people. Their decision to work for free, as self-destructive as it seems to me, is one I can only assume they made of their own conscious free will. They must expect some untangible reward for the work they do. So am I robbing a man if I accept from him what he insists on giving me in exchange for nothing?