Closing is intended to be a temporary state except in very specific circumstances (like duplicates that we keep for searchability). The end result is that there are two ends to a closed question:
- It gets reopened, so the votes would need to be valid anyway. Users may even change their votes after edits that lead to reopening.
- It gets deleted, which means that the reputation impact of votes can be removed by a recalc. The record of having the votes, however, remains useful to the system even on deleted posts.
As such, we allow votes because the close status isn't meant to affect the nature of the question itself. Closed or not, a helpful question is still a helpful question, while an unhelpful question is still an unhelpful question.
Votes can even be used to indicate which direction it should go - upvoting a question that gets edited into shape can help give it visibility, while downvoting a question that mustn't belong will help remove it from the front page.