Professionalism. Big fancy murals take time to do and requires either the artist or a group of artists to stand on platforms to do. Somebody would notice if some guys are doing such a thing illegally. Illegal graffiti is almost never as fancy as those murals.What is the distinguishing factor? The permission, medium, style, quality or what?
EDIT: As far as permission goes, if there's permission, then it's a mural. If there is no permission, it's graffiti.
Also, regarding ugly gray walls: ugly gray walls are nice. Gray isn't too nice, but plains walls are. Do you live in a home where the walls are covered with crazy stupid crap colors? No, you most likely live in a home with plain ordinary blandly colored walls. Crazy stupid crap graffiti colors are an attack upon the senses and hinders relaxation. So obviously, plainness can be (and is) aesthetically pleasing. Sometimes you like crazy bullcrap colors, sometimes you like plainness. I feel pretty certain that most people would prefer most surfaces to have plain colors rather than explosions of colors all over. And by the way, most graffiti is tremendously ugly to me.