Typically because it's based on recorded evidence rather than unfounded suppositions by people who were nowhere near the actual event.
I called you an idiot because I can see no logical grounds to assume that there's anything more to this incident than a crazy person mowing down innocents. It's not like this is the firs ... has happened or anything. The US has a serious problem with mass shootings, for reasons that are an endless subject of debate. (I have my own ideas as to why, as I'm sure does everyone else here.)
You can say that everything (truth, evidence, etc) is subjective, but that's not a very useful way to view the world unless your only goal is to muddy the waters and propagate theories that lack rational grounding. It's the same tactic that Christian conservatives and such use to argue that the earth is 4000 years old. I understand the urge—I come from a liberal arts background where I was strongly encouraged to doubt the truth of accepted facts and consider alternative viewpoints, but skepticism can be a tool for disinformation as well as truth-seeking.
Let's take an alternative, Occam's razor approach to this cop's statements: He's emotionally shaken by the shooting, he (and others who live in the immediate area of the shooting) see people on the internet propagating falsehoods regarding to what happened there, and in his emotional state he makes a foolish threat that he can't legally back up.
One guy's emotional response to a couple dozen schoolkids getting gunned down in his community, or a national/worldwide conspiracy to limit first amendment rights to free speech? Yeah, must be the latter.
You're right, I can call you an idiot for this. And I do. There's plenty of evidence that it was in fact a passenger jet that hit the pentagon if you'd like to open your eyes to it, but you won't. Sure, there are just as many resources that you can find arguing that it was a missile, or an alien ship, or a bomb. OMG, who can you trust?! Obviously no one, right?
If you disbelieve everything you don't witness with your own eyes, you must live in a very different world than most of us. You think you're enlightened because you choose to believe something different than the majority, but that's an error. You're unenlightened for ignoring evidence and logic in favor of conspiracy and paranoia.
Sincerely,
The Sheeple