It's maybe a little bit early to ask that question, but it can occur, that questions are asked, which are off-topic, and a tag is created therefore. Example: How could Aang stay alive for 100 years? avatar.
As discussed in the question, avatar is not topic of AaM, hence the question is closed. But what should happen to these tags? If I see the tag, I think "Hey, there's a tag for it, so I can ask!". Should be anything done about wrong/off-topic tags?
Ideally, an off-topic question should later be deleted. Once it is deleted, the tag becomes orphaned and will soon after be removed by the system.
I don't think it is a problem, though I can see your point.
I doubt that the presence of a tag will force people to use it. Think about it, what does one do when he wants to ask a question? First of all, he tries to search for if the question already exists. Lets assume it doesn't. Now, one will press "ask question" button. After that he creates a title, writes the question itself, and only then he goes to the tags. DO you really think that after doing all that the absence of the tag in auto-completion list is going to stop him? Unlikely.
I personally agree with the point expressed in this answer on meta.math. It states that there is a tagging problem only if the tags are inaccurate, while if the tags are accurate, there is no problem, no matter if the tags (and the question itself) are on-topic or not.
