Agree with @iant. You are paying for hosting. With the free ones, you have to download the installer yourself and provide your own machine. With the site you linked, they provide the server.
The editions differ in the number of CPUs, RAM and backup storage available. You can view the differences by using their product comparison feature.
As for charging for QGIS, they will be hosting it on their server so I guess that's what they're charging for.
What is the difference between a paid version of GeoServer or
MapServer and the free ones? Also noticed that site is charging for
the presumably free QGIS which I don't understand either.
Usually, you're not paying for the software, you're paying for support. These includes but is not limited to bugfixes, customization and training. They may also help you with the configuration and deployment to production.