I partially agree with the sentiment here (once it's closed there's no reason to keep piling on downvotes just because they're free), but I think that's a human problem - not a technical one.
I believe the reasoning behind allowing votes on closed questions is that the author can improve their question, thus making it worthy of re-opening (or the question may have been closed in error).
In those cases the upvotes a question may earn can be an indication that we should consider re-opening the question. Also if a question is edited people can change their votes on the question while it's still closed (which is particularly important for people who can upvote/downvote but can't yet vote to close/reopen).
Questions can also be locked, which prevents voting (or any other changes), but that's a moderator action (and one we tend to use with at least a little restraint since it means the OP can't fix a bad question)