I think the phrase "otherwise-ungrounded" is likely a mistake. People (and animals) conflate justice in the sense you describe of "set of subjunctive criteria" as well as justice in the folk sense of "these are the things which are a priori wrong and deserve punishment regardless of one's society". Most useful descriptions of justice need to combine and conflate these two (among other) senses into a coherent whole. Without such a combination phrases like "unjust law" become difficult to explain.