I think the point is that once you sold a copy of your free software once, the buyer may give it away for free. Realistically I'd say it's not really possible to make money by selling free software. That's okay if you can make money off from services and integration but selling isn't really possible and some free software advocates tend to ignore that fact. Especially if it's about other digital content like music. I don't think that Wu-Tang Clan's model of just selling one copy will very be sustainable.