It's the actual events as they unfolded on 9-11, it's not commentary, it's what people were saying when they say the planes hit, before, after and during the buildings falling.
People are acting like some announcer is coming on and laying out a stream of profanities. That is not the case at all, instead it's an uncensored look at what took place that day.
I see no problem with it as long as it isn't contrived.
The documentary probably went through a lot of editing unless we see it the entire recording. They could pick and choose what to see and what to hear, and they might have felt more comfortable showing those scenes with folks cussin' instead of those with people praying (if there were any.) So, the editing could make it a bit contrived.
But, in sum, if the scene itself is not contrived, and 9/11 was not contrived, then I would not oppose showing it.
Finally, I think continually showing 9/11 is a good thing. Our people seem to have forgotten that we were attacked and 3000 of us were killed.