They let just enough noise in so that you are still aware of your surroundings.
With my iPhone earphones I have to have the volume at 80% to hear them clearly over the highway/helmet noise. With the Shure earphones I have the volume at about 25%. Much better for my hearing methinks.
There's no law against driving a vehicle while deaf, that I'm aware of, which makes laws against headphones kind of silly.
And of course, headphones don't take away all the noise, they just make long hauls less tiring so you're more alert on the bike. When you ride you still need to keep your head on a swivel and keep track of what's going on around you visually. By the time you hear a problem you didn't see you're already screwed, in my opinion. But this is one of those areas that can be debated forever.
The solution is to ban motorcycles and cars and any other human-piloted vehicle, but I'm not sure that would be welcome here.
Well, the point of having in-ear monitors is to not have loud music but music at comfortable levels, and I agree that talking on the phone is a bad idea, in a car or on a bike. Which is why I don't have a communication system in my helmet, I neither want nor need one. But doing an Iron Butt with no music, I don't think that would increase my safety, I think it would decrease it.
Riding without earplugs at least is just not smart. It will screw your hearing up, guaranteed. If it's already screwed up then perhaps adding more damage is fine, but I prefer to baby my ears at least.