The (understandable, traditional) reason for measuring relative to placebo is that, "well, you don't need the drug if it's only as good as what a placebo gives -- just use the cheaper placebo! You should only bother with some special drug if it's better than what mere belief-in-effectiveness yields."

But that assumes that people are equally credulous of every placebo; if a person is more credulous of homeopathy, and credulity is hard to manufacture, then it's not quite wrong to say that it "works" under a relevant definition.

(I know this is kind of an angels-on-the-head-of-a-pen technicality, but eventually we'll want to actually understand why belief-in-effectiveness can matter at all, so we can exploit it, and reasoning like yours tends to give the idea that such an effort, by definition, can't work.)