If People Say Graffiti Is A Kind Of Art Than Why Do They Call It Vandalism?

Have you ever had to wash spray paint off a brick wall? Its time consuming and a pain in the a$$. I personally live by train tracks and i like seeing it but ive also had to clean it as well

Not all graffiti is vandalism.  Damage to another person's property is vandalism.   Graffiti on property that is permissible by the owner is not vandalism.  On the same note, not all graffiti is art.  Tagging and spraying obscenities is not art, it's just some punk with a rattle can spraying a mess on something.   My dog does that when he sees a bush or a tree... I don't consider that artistic either.

Just because something is artistic, doesn't make it legal or morally correct. Renaissance artists robbed graves and preformed illegal vivisection on corpses to study musculature and skeletal systems of the human body in order to sculpt and paint them with better accuracy and attention to detail.  Many artists use illegal hallucinogenic and mind altering drugs to influence their art.  

The type of canvas makes it vandalism.
Giving permission eliminates the art as "vandalism".
If you were a great painter and just decided to hang your art in any museum, the same could be considered. Really people love their concrete buildings so tagging is considered vandalism, because no permission was given and irremovable art drives down property values.