Absolutely nothing says you can't sell open source stuff. Look at RedHat, Ubuntu/Canonical.
That's true, Canonical has sales & support offices all over the modern parts of the world, specializing in custom OS's for most any business one can think of, be it the financial sectors (think banks & online transactions), government agencies of all types worldwide, and even physician's offices where there's not enough seats to qualify for W10 Enterprise licensing, running Pro is not the answer, would violate the HIPPA act (in the US), because of the uncontrollable uploading of sensitive data through the hardware, only offers more features than the Home license, doesn't stop the uploading of data. That function is only available for Enterprise subscribers.
That's where the subscription based Linux versions comes into play, tax accountants has been running Red Hat for years, because they save lots of cash in reusing the same computers for many years. Of course by now, most are on 64 bit hardware for speed, security that only the 64 bit CPU & OS can deliver, and I personally feel that Canonical, as well as the distributors of Red Hat deserves to earn a living. Remember, they're not 'selling' the OS's, rather subscriptions & more important for business, service after the sale.
If one wants a 'free' Red Hat, look no further than CentOS, which is the testing bed that the developers uses to improve their bread & butter distro. Some of the software is actually good, just doesn't make the final cut into the next Red Hat release, so one can see the free CentOS as more than a testbed, if they want to use it w/out support, that's fine & permissible for personal use (not sure about business, yet what's there to stop them?).