Actually, screaming is a primordial and very common across the species.
Screaming evolved as a survival alert, not for the screamer but for the rest of the group to high tail it out of the area there is life threatening danger. IDK, I made that up.
Unfortunately, you can't tell much from the video. It would have been much better had the camera been secured to a part of the plane. That way, you could see things moving around relative to the plane. As is, it's not so different from if someone had recorded the cabin while waving their arm around.
This falls on AA Flight Dispatch sending this plane through the winter storm with tailwinds predicted to be 250kts or greater and a Tokyo Sigmet for Upper Air Turbulence. Almost all other flights went north almost polar. AA could not help themselves with the thought of cutting an hour or ore of the flight time. How did that work out?
Im going to guess that this was some very heavy CAT and that wouldn't be on their charts. Maybe they could have flown through it or even requested lower, but with injuries they had to divert to NRT. I could be wrong though.
I've been in similar extreme turbulence crossing the Pacific on a JAL 747-400 flight from LAX to Narita. People were also scared and screaming. I was reading the newspaper. What can you do, when you are at cruising altitude and in the middle of nowhere? Nothing at all, so I just continued to read the newspaper and shortly afterwards the turbulence subsided. Either the majority of those people screaming had not flown much, or perhaps, I have flown too much to worry about it.
It appeared from the video that the plane was shaking a lot, but there was no threat of a crash. For turbulence to be "extreme," it has to be rough enough for the pilots to loose control. Since that didn't appear to happen, the turbulence was, at its worst, only "severe." Bad headline!
It is natural for those in fear for their lives. When in an "exciting" situation, the natural response is to yell or express the discomfort being felt. For some, such discomfort is terrifying (like this flight) and for others, it is taken as joy (like a roller coaster ride.)
Gives you a sample of what it must have sounded like in the back when Alaska Flight 261 went out of control and crashed back in January 2000 as a result of a screw jack actuator for the horiz stab on the MD-83 disconnecting in flight in what may have largely been the result of its Captain deciding to become a test pilot. Same could be said for AF447 as it sank toward the black Atlantic ocean from 30,000 feet at night time in a deep stall as a result of the total incompetence of its crew who refused to apply fundamental stall recovery techniques because they blindly trusted their FMS computer rather than the seat of their pants as the airplane spoke to them repeatedly saying "stall, stall, stall" while buffeting and wing rocking violently. Where do we get such men? Did this American crew ever slow down in the midst of what would appear to have been severe turbulence? No way to tell from this video. Never heard any PAs either though there may have been one or more outside the range of this brief video.
No way to verify that you are the ATP genius that you depict in a narrative designed to question the pilots competence in scenarios you have never encountered.
Turbulence penetration speed is almost the same in most airliners, around M.08 which is close to cruise speed. As for AF447, the airline did not cover the scenario in their training and the aircraft manufacturer allowed operations with known defective parts on the airplane.