What if Linus said, "I have this great idea for a version control system, but I need the community to raise $100k to make it happen"? Of course he would never do that. The reason for making git was to give the kernel project what it needed for a version control system. Linus' time is at least an order of magnitude more valuable than Yahuda Katz' time. Just as git arguably makes the kernel project more successful, rails.app ought to make Rails more popular. Wide deployment of Linux in turn makes Linux developers' skills more valuable, just as wider deployment of Rails enriches Ruby developers.
c) ... Rails was designed to, and succeeds at, making companies great piles of money
d) OSS developers making money by it
That can be said for just about any popular open source project used in the business world.