Red Hat's money comes from support subscriptions, which are all provided by paid employees. Red Hat doesn't use volunteers to provide support to customers. Volunteer work is done at many levels (linux kernel, GNU libs, GNOME, Fedora, etc.) but Red Hat does not make money from those projects directly (no more than any other company would/could). In fact Red Hat employs people to just work on those projects, and gives all the output to the community for free to benefit everyone.

Disclaimer: I work for Red Hat but I'm speaking as myself, not as a Red Hat employee