Well, let's take it further.   Why have any laws?    Vigilante justice would prevail and take care of things naturally, right?    

At the very least, a distracted walker law would put some of the responsibility on the distracted pedestrian when Darwin's law kicks in and they are hit by a car.    No, it won't be 100% their fault - but then it won't be 100% the driver's fault either.     Win win would be if the distracted walker were hit by a texting driver - so they're both off the streets.