“Defeasible” (i.e., defeatable) is a legal term metaphorically used by Brandom for anything provisionally affirmed or accepted. The great majority of things we affirm or accept fall into this category.
Brandom applies a legal model of due process to ethical negotiation. We start with presumptions of innocence and good faith. This means that until a cause for reasonable doubt is shown, we act on the assumption that people have good reasons for what they say and for what they want; but anything can be challenged. (See also Things Said; Dialogue; Assumptions; Reasons; Desire, Coherence; Commitment; Mutual Recognition; Scorekeeping.)
In general, I strongly believe that ethics should drive law and not the other way around, but I would argue that the broad notion of due process actually is a good example of ethics driving law, as it should.