People slap the label "art" on crap too quickly. No, not all of it is art. Some of it is; but not all. Just because you can sling some pigment against a flat surface doesn't make it art, and just because you can draw disapproving looks from people doesn't make you an "artist".
Fucking up Marc Jacobs or other massive commercial brands is a cogent statement to be made, and i don't think one that should simply be dismissed as thoughtless vandalism (though vandalism it may be).
The artist intended it to be art, and had a message to convey. Those are definitely components which make me lean towards considering it art.
Not claiming this particular graffiti is art - it's rather uncreative compared to Banksy and other such clever artists - just wondering who decides, and how.
Also, good for Mark Jacobs for his elegant response, and for trolling the pretentious.