Only while the sun is up; and even then, the sky is there, but you actually look at everything else for the vast majority of the time.
If the ground is as bright as the sky -- for example, if it's covered in snow or water that reflects most of the light hitting it -- then that's quite a bit more dangerous than watching a screen all day (see snow blindness).
Just because things are "natural" doesn't mean they're automatically safe, or good for you.
They forgot number 11 -- never go outdoors or look at the sky, saturated as it is with that harmful blue light.