While it's true you can charge for your OSS product, all OSS licenses allow the recipient to distribute the source code (and the built binaries) at no cost. So, OSS does imply free, even if it doesn't mandate it.
I'd pay for a distribution if it was better then free (cost) software.
For instance, if Apple decided today "Hey why not just make OSX a set of applications, a window manager, and a desktop environment for Linux and make it free software." I'd pay for it. I'd pay the 100 bucks because no one really does UX better then Apple.
While it's true you can charge for your OSS product, all OSS licenses allow the recipient to distribute the source code (and the built binaries) at no cost. So, OSS does imply free, even if it doesn't mandate it.