Why? I'm not trying to rely on SEP as an authority here. Indeed, given your own findings about how the terms are used in the philosophical literature, no such authority could plausibly exist.

What I can hope to show is that my use of the terms is more meaningful than others' (it "carves concept-space at its joints") and that it's at least roughly consistent with what English speakers mean when they talk about "morality" and "ethics". AFAICT, the SEP entry supports my argument: it exemplifies a meaningful distinction between "descriptive" and "normative" morality, which most English-speaking folks would readily describe as a distinction between "morality" and "ethics".