Why do gravity and electromagnetic radiation both follow inverse square laws?

[EnumaElish]

How does inverse square law of gravity relate to the inverse square law of electromagnetic radiation (e.g. illumination)? How come gravity and EMR propagate identically even though they are different forces?

 

[mgb_phys]

Think of anything that spreads out from a point into a sphere, the force is spread over the surface of the sphere - and the area of the sphere depends on the radiius squared.