In the tutorials, user sharing is done through templating of slave 1. Can users be shared directly from the master site? Is there a specific reason this is not shown?

YES you can share the users with the master.
The reason why I have not show that is as we explained in the tutorial video 0.b slide 21
www.jms2win.com/tutorial#jms12x-0b
we consider the master as a repository or a library where you install all the extension for all the slave site.
So for us, the master website should not have any users and should not be public.
We prefer to recommend that your main site or portal or corporate or whatever would be the "slave1" website as shown in the tutorial video 16.
But this is just a recommendation (a proposal) and you can do it in another way.