In this question I am asking why we use the negative (isn't it/that), as if we are challenging their point of view, rather than the positive (is it/that), as if we are expecting them to agree with us, when we are asking for confirmation of a statement/asking a rhetorical question? An example of this is; 'wearing makeup everyday is pointless isn't it?' That was a bad example I admit but I think you know what I mean.
It's one of my pet peeves. Asking "isn't it" when asking for an affirmative response, is actually asking, "is it not", to which the proper response would be "no", if you are in agreement that the statement is true. If you answer in the affirmative, then you are saying it is not so. One should ask, is it true, or is it so, or do you agree, but not don't you agree, since that is asking if you do not agree.