Voting arrows do not mean you can vote on the post. If they did, that would mean we'd have to remove them:
- when you're not logged in
- when you don't have enough rep to vote
- when you don't have enough rep to downvote (the down arrow would be removed)
- when you are post's author
- when you have used all your votes for the day
Voting arrows are shown in all these cases and, as animuson says (though not in these words):
Most locked questions have voting arrows.
When a question is locked because it is only kept (rather than deleted) for "historical interest," that is when the arrows disappear.
When the voting arrows disappear, that signifies: This is not a normal post. It is not even a normal locked post. It was voted on once, but will never be voted on again.
This situation applies to none of the situations listed above (e.g., when you're not logged in). It also does not apply to most locked posts--posts that are locked but not deleted, and not locked for historical reasons, are usually only temporarily locked. It makes sense to think about voting on them, and you likely will be able to vote on them in the future.
Thus, posts that are kept only for historical interest are qualitatively different, and the system's current behavior of removing voting arrows just for them makes sense. The way things are is far less astonishing than if a post with no voting arrows today could be voted on tomorrow (or whenever it is unlocked).