I want to know, Why peole called graffiti an art.?

I am very intrigue with the idea of calling graffiti an art. I belive that graffiti is a bad influence to young people because they do not respect peoples properties. This so called artist do it in places that they do not own. specially busses, trains,trucks houses. to me graffiti is a from of vandalism. The money the cieties expend on removing it could be use to make schools better.