You haven't understood a word that we've said. The proposition "people give reasons" isn't the proposition that Habermas is defending. He's defending the pragmatic-structural logic of communication such that it can serve as the ground for social criticism. This isn't a tautology. You really should read Habermas's essay Discourse Ethics in Moral Consciousness and Communicative Action. At least then you'd be informed about what you're talking about.
I'll try to put this as carefully and concisely as I can:
Without a normative foundation, social criticism has no carry - it can yield neither reasonable nor rhetorical weight without some sort of behavioural expectation that goes along with it. Normative expectations are implicit in anything that you say, whether you intend it or not. If you deny this assertion, you end up contradicting yourself, because understanding is normative, wholly aside from the propositional normative content that practical discourse posits (ie. If you say that we ought to love one another, there is the normative expectation of understanding *and* the normative propositional claim of loving one another). You comment above indicates that you think the idea of understanding is a tautology, that it is a meaningless statement. If find this dumbfounding. That you consider the single most important aspect of cognition subjectivity and intersubjectivity to be irrelevant is simply mind-blowing. It is the social and scientific equivalent of saying that everything Marx wrote about political economy is a "dull truism."
ken