The meaning of votes on meta is what we decide for them to mean. Some sites operate as said in OP but not all; or at least to quite different degrees as already said in some sense by Logan Maingi.
I certainly agree that down-votes on meta can and should be cast a lot more freely, but the "downvote express disagree" idea is in my opinion somewhat problematic for several reasons (I only give the most clear-cut one) especially when combined with a call to vote a lot.
Suppose I write: "I think we should do X only in case condition Y is met."
Then A thinking we should do X always and N thinking we should never do X, both disagree with me while having completely contradictory views.
So what a downvote on "I think we should do X only in case condition Y is met" really means is anybodies guess. (This is not some contrived example, I got dv on posts I consider middle-of-the-road positions where I had and have no idea in which direction the dissent was.)