Why? Maybe he was paid by the linux foundation during that time? Other sources of income? The gratuit "of course" is not clear to me.
And Rails is sponsored by 37Signals. You'd be hard pressed to find very many large open source projects without commercial backing.
Are you saying that Yehuda should not make money from his efforts, but the Ruby developers in turn can use the results naturally for their gain?
That is generally how things work. Companies specializing in open source technologies have a business interest in their specialty software(s) becoming more popular. A core Rails developer will command the highest speaking and consulting fees.
I think the reservation many people have about all of this is that it feels backwards. Asking for sponsors is one thing, but making the open source project pay-to-play flies in the face of the spirit of FLOSS.