I am not sure exactly why, but there might be some difficulty in deciding whether to show the tag when it’s only used on restricted categories. I think if the Include secure categories in tag counts site setting is enabled, tags on restricted categories appear for everyone.
But I think a tag will also be visible in the tag list with the correct count based on your permissions if the setting is disabled and it’s applied to at least one topic in a public category. So, I am not sure why it’s hidden if it is used only on restricted categories.