Let's petition the few model engine manufacturers that are left that still actually make engines to put a "common sense warning" on their boxes and in large bold print in the manual that running model engines may cause hearing damage. Add that to the area where they caution about not running engines near other people in the event someone sticks their fingers in the prop arc of if the prop isn't properly fastened and can come flying off, or model engines can cause burns, or the fuel is poisonous, etc.
Come on, it's all about using common sense (and common courtesy if others are nearby) and not being an idiot. If someone runs an engine sans muffler without hearing protection, they're idiots and deserve the possible hearing damage that may result. That has nothing to do with any safety standards other than if you violate noise regulations at your club field or in your city, county, etc.
For the record, I always run my engines with hearing protection if on the bench; muffler or not.