The popular adage “everyone has the right to their opinion” is a partial truth. It gestures at something like Kantian autonomy. But the other side of the coin is that Kantian autonomy does not operate in a vacuum; it is always embedded in what Wilfrid Sellars called the space of reasons, where holding a commitment is implicitly to invite questions about the reasons for it, and then about the actual validity or applicability of those reasons. As ethical beings we are responsible for our opinions.
An attitude in the ordinary unqualified sense may operate as a cause of behavior in the modern sense, but because nothing prevents such an attitude from being completely arbitrary, a mere attitude cannot serve as a justification for anything. Attitudes in this sense have a sort of vain Cartesian irrefutability, in the same way that mere appearances do. There is a level at which if you say something appears to you thus-and-such, that mere appearance is incontestable. But it is incontestable precisely because nothing follows from it.
Brandom, on the other hand, is only interested in attitudes from which something is taken to follow. When he talks about the attitude-dependence of norms, I think what he really has in mind is more specifically a dependence on what analytic philosophers call propositional attitudes. This additional specification is crucial. An endorsement of any proposition whatsoever can still qualify as a propositional attitude, but every proposition has a “place” in the space of reasons that situates it in a multi-dimensional spectrum of goodness of justification. Criteria for goodness of justification can of course be debated too, but this still means that the justification for any propositional attitude is subject to evaluation. We may judge a propositional attitude to be poorly founded and therefore wrong, but it still cannot be completely arbitrary, because its justification can be evaluated.
I don’t think the same can be said for the Enlightenment authors Brandom cites as promoting a dependence of norms on attitudes, like Pufendorf and Rousseau (see Modernity, Voluntarism; Modernity, Rouseau?). Their legal and political voluntarism explicitly purports to trump any evaluation in terms of the space of reasons. This is in spite of the fact that those authors were already departing from the most traditional notion of ethical norms as simply somehow pre-given. Precisely because I am deeply sympathetic to Brandom’s critique of the unilateral authority-obedience model, I disapprove of these authors’ appeals to sovereignty.
Brandom shares his mentor Richard Rorty’s concern for democratic values, and suggests that normativity be considered as a historical development. But in spite of his concern for “semantic descent” to relate the higher-order, overtly philosophical concepts Kant and Hegel focused on to the level of ordinary empirical concepts used in daily life, he still treats ordinary concepts in a general way. Consequently, when he thinks about ordinary concepts, he still does so in a way analogous to that of Kant and Hegel. This is not a bad thing; I think he is by far the strongest at this high level. Fortunately, the weaker remarks about concrete historical antecedents for his views are peripheral to the main development of his thought.