Coming from an undergraduate background in academic philosophy (a.k.a. "useless wanking"), as an articling student, I idly wondered what kind of claim I was making when I made one of these utterances. I was inclined to divide the universe of propositions into positive statements of fact and normative statements of value (or theoretical and practical judgments, or isses and oughts.) Am I telling my clients certain social facts about what consequences will likely flow from what they want to do, or am I reasoning about what right requires?
Looking at this from the articling student's point-of-view, neither alternative sounds attractive. Most articling students -- because of their age, cohort and class -- think marijuana should be legalized. But if a client asks whether it is lawful to expand their product line to include THC brownies, they know that it isn't. On the other hand, ethical articling students also know that they are supposed to tell the client what the law requires, even if it is unlikely that the client would get caught for doing the opposite. But if we take Holmes' bad man theory of the law seriously, then an unlawful act that is never detected just is a lawful act. The Law Society will take a different view.
Or look at it from the justice of the final court's point-of-view. When they opine what the law is, they can't simply be stating a social fact about what will be enforced. Unless deciding one way would trigger a constitutional crisis with another or revolution, then whateve they decide will be enforced. However, at least in most cases, final court justices don't believe that they are supposed to just do whatever they think is right. There is some sense in which a final court could act lawlessly, and they accept this, at least in principle. Indeed, judges are never so comfortable as when they can say that the way they are ruling is contrary to their personal preference as to what the law should be.
In the end, I think a statement of legal opinion is a normative, not a positive, utterance. But it is a kind of normative utterance that gives more weight to authority than we moderns are inclined to do for other kind of practical judgments. The legal positivist is right to say that we are inclined to deduce a lot of legal rules this way:
(P1) X is authorized to proclaim the kind of rule Y;
(P2) X proclaimed rule Y;
(C) Therefore, rule Y.
But P1 is just as much a normative proposition as C. Legal positivists who imagine they have a value-free system miss this. P1 is therefore defeasible. At some point, X may be exceeding its authority, and must be resisted.
Of course, any body of normative utterances can be described positively from the outside. This is what anthropologists try to do with their notorious "cultural relativism." But that's actual a difficult stance even for anthropologists to maintain, and obviously anthropologists -- to the extent they take their cultural relativism seriously -- are not supposed to participate in the development of the culture's normative thinking. Lawyers and judges are so expected, so they can't ultimately take that point-of-view.
I suppose that a revolutionary lawyer (i.e., one who denied the authority, but not the power, of the official sources of law) could be successful, but only in the way that a sociopath might be able to figure out how normal people will react morally. In each case, the outsider would have to simulate in his or her own mind how insiders think. In some way, though, the revolutionary lawyer's statement of legal opinion would be in bad faith, while those of a lawyer who would like to see the law in question changed, but accepts the overall system of authoritatively stating law, would not.
This sequence of thoughts leads me to a natural law perspective. I'll try to defend that next time I get the energy up for such abstract posts.
 I simplify. If you don't like it, take it up in the comments.
 OK, not quite "therefore."