Come to think of it, how does a guy with force-beam-emitting eyes cry in the first place? Wouldn't the tears be instantly evaporated?
Well, you might as well ask why his power stops working when he closes his eyes. Are his eyelids indestructible or something? Doesn't he just bake his eyeballs? Personally, I think his tears probably contribute to soil erosion or something; it's probably a good thing they killed him in the film.