It's exceedingly rare that someone gets hurt due to turbulence, which has no bearing the particular subject the OP asked about hear, which is wearing seatbelts on takeoff and landing. But, for someone who is hurt due to turbulence, at altitude, how many people have been hurt due to another passenger not having their seatbelt on?

You can of course find corner cases, but it's an extremely remote possibility that you will be harmed because the fat guy a few seats over didn't put his seatbelt on, or the skinny guy a few seats over decides to take his off once in the air.

My main point - this is nitpicky stuff, and something that shouldn't offend any passenger. If it does, then the passenger should just keep their overbearing mouth shut and silently deal with their rage against all things non-comforming. If everyone piped up every time someone was technically breaking a rule - a carry-on that's too big, too many carry-ons, not turning a phone off, putting something in the seatback that is forbidden that day - then none of us would ever get off the ground.

These types of questions come up every couple of weeks. "I saw him not turn his phone off, should I tell someone? I saw him pull out a liquid bigger than 3 ounces, should I tell TSA? Blah blah blah". This stuff is just too stupid to care about. We flew for decades with our seatbelts off once above 10,000 feet and the air was clear. We flew for decades with bottles of water and big cans of hairspray and shaving cream. We all survived, and were happier traveling then than now.

Put it another way - if everyone starts ratting each other out on small, inconsequential things, that's the day I start asking for tomato juice as my beverage, and the day I suddenly develop Parkinsons and accidentally start spilling tomato juice all over my fellow rat-fink passengers.