I recently edited in a tag for a question, answered it, got an upvote on the answer, but the counter for "Illuminator" didn't go up. Tags are part of the question are they not? Hence you are editing the question. It seems like simple logic.
Strictly-speaking, tags are meta-data; they help to organize questions by extracting a few key topics or categories from it, but are generally separate from the question itself.
Contrast with the title (which can and should summarize the question) and the body (which should contain both the full question and whatever data is needed for context), which together can be said to make up the question as a whole.
That's not to say that editing tags isn't useful - it is - but from a practical perspective the purpose of these badges was to encourage folks to make much more substantive edits when answering questions, using their knowledge of the topic to flesh out the nature of the question, highlight key points, and generally just make the question more accessible to others who might come along and benefit from it (and the answer). As helpful as retagging can be, it can also be done by just about anyone; writing a good title or honing examples needs the expertise of someone familiar with the topic.