Why not give a nice simple +1 rep for retagging?
At the moment there is no incentive.
When an edit (to tags or anything else) requires approval, and is approved, the editor gets +2.
When an edit (including to tags) doesn't require approval, there is no real reason to think it necessarily was good. Automatically giving any reputation (even 1 rep) for activities that don't involve the approval of another person is rarely done, and when it is, the amount of reputation that can be gained this way is strictly limited. The only good example of this that I can think of is the +100 association bonus. Even that is reliant upon having gained rep elsewhere (so even this bonus flows from other people's evaluations of one's work).
Therefore, this feature should not be implemented. To incentivize editing irrespective of the content of the edit would be actively bad, because it would encourage unnecessary or wrong edits, or at least would make it harder to discourage them. This applies at least as much to tag edits as other edits, since what tags a post ought to have is often quite subjective.
You say "[a]t the moment there is no incentive." That is not true. There is the incentive of the post getting the tags you thought it ought to have. There's a reason why people edit even when they don't get reputation for it--they want to make the site better (according to their idea of what is better), and editing makes this happen.
We might call this an "intrinsic incentive" and you might say you're specifically talking about "extrinsic incentives." Well, it turns out there currently is also an extrinsic incentive. Tag edits appear in your list of edits; it is recorded that you did them, and when people look at your profile, it's there. Imagine if your profile stopped showing your edits. I can't speak for you, but if that happened to me, I'd not like it very much. Which suggests I find value in my edits being listed. Similarly, there's no reputation incentive for community wiki posts, but I and many other users contribute to them.
Furthermore, my understanding is that tag edits do qualify as edits for the purpose of conferring all editing-related badges. I admit that I'm not entirely sure about that specific point and another user questions it in another comment there. However, this is a highly peripheral part of my argument, because if this problem hinges on insufficient conferral of badges, the solution is to extend existing badges or make new ones to provide the extrinsic incentives for tag editing that you're requesting, without introducing rep boosts just for changing content. I'm not convinced any additional badges are needed, but even if they are, that's a different feature request from this.
TL;DR: Users should only get reputation for contributions that other people have voted on, whether by voting up (for posts), or voting to approve (for edits). We don't edit for the sake of editing; only edits that improve a post are good, so edits where no other user has expressed anything about the post should not give reputation to the editor.