When I talk about Meta here, I mean any meta but this one.
I understand the Idea behind “Your vote is now locked in unless this answer is edited”. However, I don't think this reason applies to meta sites and have some reasons why I think votes on meta sites shouldn't get locked:
I thought a feature-request was just stupid and I downvoted it. Later on, after reading the answers, I saw the ideas behind it (which weren't included in the question), and wanted to upvote it - but I couldn't. Of course, the question was written badly, but ...
Votes on meta work in a different way,
as they indicate (dis)agreement instead of usefulness. So in my example, the question wasn't really useful as-is, but I did agree, after hearing the ideas behind it.
You don't get reputation on Meta,
so there is no reason to tactically downvote1.
What would you say of disabling the lock-vote feature on meta sites?
There is no problem with users who go bumping posts by up- and downvoting all the time (as suggested in the comments) since votes don't bump - thanks Hugo and cpast.
Some very clear examples of when you want this
Moderator pro tempore elections - what if a nominated user does something stupid and you want to remove your upvote? Nope, too late.
Feature-requests can get more useful after a new feature implemented (e.g. when there's a request for changing the footer that you downvoted, and then the team changes the footer in such a way that you want to upvote it instead)
1: as Shog9♦ points out, there still is a reason to tactically downvote: when you've answered a question yourself as well, then downvote the other answers to get your answer shown higher. I don't think this is a real problem with this feature-request, but a problem we have now too and will probably always have.