Body mass index is a measure of weight, which isn't quite the same thing as obesity; and the real health hazard is obesity - too much fat. Here's a nice summary of some if the issues.
A very long time ago (i.e. I can't track down the reference) I read a short story by HG Wells. A fanastical inventor advertised a machine which guaranteed weight loss. It turned that this promise was literally true, and the story ended with a whole load of people, still very fat but now weighing nothing, floating around on the ceiling.
As mentioned the other week, accuracy of speech and word matters in science!