I'm stunned that anybody sympathetic to him could claim Marx rejected moral talk. To my eye, every letter of every work he ever wrote is all about ethics. To my mind, his whole project was to show why capitalism's claim to be the end of history was bogus, and that's ultimately a moral claim about the best of all possible worlds.
Beyond that, what kind of madman thinks consideration of justice is detachable from consideration of ethics? Justice is a wholly ethical concept, is it not? I mean, go on out in the street and start up a conversation on the subject. "I want to talk to you about justice, but this is purely a technical question." That's insane!
Of course, the first utilitarians rejected talk of justice! They short-circuited the application of their own model by presuming that they already lived in the best of all possible worlds, that letting the poor vote would ruin it, etc. And they were wrong -- shockingly so, given what they argued about micro-politics. If you say (rightly) that the best of all possible worlds is the world that maximizes happiness for the maximum number of people, then logic compels you to a pretty strong and clear theory of justice. That Bentham et al. stopped themselves from talking about this merely shows how partial and inadequate they were. As none other than Herr Marx, cuttingly, noted...