Ah yes, the tired old argument that tries to make arbitrary barriers onto human expression. Tell me, what does the Mona Lisa mean to you? What about it fulfills artistic requirements that Postal lacks (forethought, expression, message, form, craftsmanship, continuity)? Or are you merely buying these simplistic socially enforced walls and the closed-minded prudery promoted through social conditioning simply because you dare not think of it on your own? How is the Postal Dude's nihilism all that different than Hamlet's musings on death during the famous graveyard scene? How are his observations on all that is wrong around him any different than the Danish prince's?
Stop being so up yourself that you tear down a person's work simply because it does not fulfill your own narrow-minded rules. There is more to art than reputation and patronage, and you are being too pig-headed to grasp it.
EDIT: By the way, the ability to offend is an artistic quality just as much as film making you sad or a painting making you uplifted. Again, your narrow-mindedness is telling.