The Tag List is simply a List of Tags currently in use (in at least 1 book).
Once you remove the last usage, it is gone from the list.
Simply add the name back to Tags on a book (Best it apply to that book
)
Switching to what you thought you were doing:
Deleting all books that have a tag (we are keeping it simple here):
Select the tag in the Tag browser (Green +): Now Select All (Ctrl+A or click on the box just above the row numbers): Click the Recycle Icon or use the option arrow at the right edge for finer control