Correct me if I'm wrong, but isn't Linus Torvalds famous for his iron grip of Linux? In a very different way, to be sure, but it's my understanding that just because you make something open source doesn't mean you have to (or even should) relinquish control.

I think it's also pretty standard to open-source the core and keep the baubles proprietary. GitHub, for example, made their git interaction library open-source but their git hosting service itself is closed, as far as I know.