An excellent book review that sets up this question, which has been answered in various ways over the years, is here (ND reviews).
The issue usually rests on the meaning of Dharma - as it is accepted that all (Classical/pre-Classical) Indian philosophies are "Dharmic" in nature; that is, they have Dharma as an explicit Ultimate or Absolute. In Buddhism it may be a process-Absolute instead of a substantial one, but it is an absolute nonetheless. It is absolute in the sense that nothing is greater than or beyond it...
This review does a great job of arguing that we deeply question the ethical nature of Dharma, as it is a term that seems to be used in what we would today call ontological or phenomenological ways. And it would be meaningless question-begging to say that, "oh, that all is ethics too!"