It may be that the above was a deliberate decision here on meta to encourage users to choose their own categories/tags, so it may be that the defaults are empty.
Nevertheless, seeing that made me think that it would be useful for users to be able to reset their preferences to the site default much in the same way that admin can reset site settings to the Discourse default.
E.g. where the admin can see something like this: (announcement tag added, therefore title highlighted to indicate that it is not the default and a reset button is present)
The user currently might see this in their preferences:
In this example, it would be useful for the user to have a reset button which would add the announcement tag when pressed, i.e. reset to the default tags watching first post site setting.
This behaviour would be available for any user preference which has a corresponding site setting for its default value. Presumably this would compare each preference value against the corresponding site setting when being displayed and show/hide the reset button as appropriate, as well as show the button when changing to a non-matching value and hide it when changing to a matching value.