Graffiti can indeed be artistic. Art is not meant to be comfortable or law abiding, it should challenge your assumptions and make you think. So graffiti that has been done with permission is not automatically artistic in my opinion and done without permission can be both vandalism and art. The two are not mutually exclusive. Think of the Berlin Wall - here the graffiti was most definitely done without permission but that was what gave the art and the message it was conveying its power. With permission it would have been nothing more than daubs of paint on a wall. In more recent times, Banksy has made people stop and think about all sorts of issues. His work is beautiful and challenging and sometimes mind-boggling as to how he got it there! But it is art without doubt.
That's not to say that all graffiti is art, any more than all paintings on canvas are art. But the issue of having permission or whether it is vandalism seems to me to be completely irrelevant to whether it is art.
Art and artists are often subversive and mocking the current mores, if they had to have permission then we would have been denied a lot of great art over the ages.