The video of the 757s hitting the WTC isn't purged from the internet, why should this video be? The world is dark and full of terrors (but getting better every day), and sticking our heads in the sand doesn't change that.
Can you elaborate on your apparently implicit assertion that the decision of major social media platforms to ban the content constitutes "sticking our heads in the sand" or an attempt to get it "purged from the internet"? That's such a black-and-white leap to me that I'm baffled. Edit: Or perhaps the "implicit" part of your post is that you're referring to the ISP incident in NZ, and not Facebook?
That 757 video was played on repeat for weeks on all of the major channels in 2001. Hundreds of people turning into a fireball over and over.
It made it real for us.
By hiding the video from ChristChurch, people have to imagine how awful the event was. And they'll fall back to the stylized Hollywood notions of violence as a reference point. There are no heroic close-ups in this footage (I found a copy).
If you want people to get upset about this and take action, you show them the truth. It would repulse almost everyone.
So, since a very very small part of Internet consumers venture out of the big social and old media sites, by not hosting it there they will never see it.