Trash is art. Why not the defacement of public property?

I believe that this picture shows an artist's masterpiece.  Art is the expression of an artist's thoughts and emotions using any type of resource to do that.  These artists have decide spray painting a wall would convey their message.  In comparison, some artists choose to express themselves by using trash.  Trash art seems crude and unusual but it still holds an inner beauty that art is meant to show.  So I say that if trash is art, then defacing public property is.