They can't charge for extensions created by members.
They can charge for access to a well done repository. A repository you can access from your admin screens, that checks for updates, let's you peruse what's out there and installs it for you. If an extension that's made by a member happens to be on the list of available, installable extensions, does that hurt or help said member?
Now if we want to compensate members for their work via some mechanism to track installed extensions, well that would be a win-win for everybody.