I can't think of one time I have sat there and thought "golly this 24hr screeching in my ear is a bit bothersome!"
It is bothersome, but it, in of itself, is not life threatening. "Life threatening" is a medical term used to describe a disease or injury that by itself can kill you. It would be factually incorrect to use that term to describe tinnitus.
The problem is equating "non-life-threatening" with "merely bothersome." Some non-life-threatening conditions can be fates worse than death.