Why does Adorno prefer to use the term “morality” rather than “ethics” in the text, Problems of Moral Philosophy?
Adorno's practical philosophy stands in the tradition of (and in opposition to) Kantian moral philosophy: It is about freedom and rational reasons, not simply habits or social rules.
Historically, Adorno lived through Nazi Germany, where people happily doomed other humans and treated each other like animals (seemingly) without ethical concern, so that his book Minima Moralia and the later lectures Problems of Moral Philosophy urge for a stronger - less idealistic - "ought" which cannot be independent of, but still has to be both different from and based in practical reality. As such, the German "Moralphilosophie" is appropriate. To cite Joel Whitebook in "The Cambridge Companion to Adorno" (p.60):
Significantly, he begins his metacritique of practical reason with the assertion that the antinomous outcome of the Kantian construction – in both the Critique of Pure Reason and the Critique of Practical Reason – results from the original separation of (transcendental) philosophy from (empirical) psychology. According to the standard philosophical reading, Kant insisted on this strict separation to guarantee the autonomy of the moral subject and the purity of the moral law. Following the analysis of the Juliette chapter of the Dialectic of Enlightenment – where the Sadean orgy, with its instrumentalized pursuit of pleasure and pain, and not the categorical imperative, is proclaimed the real moral content of the bourgeois world – Adorno gives a more intraworldly explanation of the source of this separation of the transcendental and the empirical. He argues that Kant was fulfilling “an unexpressed mandate,” which the bourgeoisie had bequeathed to philosophy “ever since the seventeenth century,” namely, “to find transparent grounds for freedom.”