The term arsenal in my mind conjures up visions of criminals. Which of course is why the media uses such language.
How about the term is used because it's accurate when referring to a collection of guns? And shorter to say, too. You (or I) have no way of knowing why any particular word was chosen, especially when the definition is exactly as described - a collection of guns.
That said, (and somewhat to your point) whenever I see the word "arsenal" I also want to read, within a sentence or two, an accurate count of the gun collection, and types. Because the devil is in the details. So far in the Aurora reporting we've seen that, mostly, and "arsenal" is just shorthand for "collection of guns."
As mentioned before the words in the OP are in quotes so the assumption is that the person being quoted used those words. If they were trying to create a stir, then so be it. People in positions of authority often
are trying to stir the pudding (see: Current US election). And the "the media" are
obligated to report what they said - ideally as accurately as possible - without boring the reader to sleep.
News stories are not court testimony; they are written to be concise (ideally), informative (ideally) and accurate (even more ideally). At least, for written news; the "if it bleeds, it leads" gorp on what passes for local TV news doesn't seem to even try to pass the first test.