Put on your seat belt all the time, unless you are leaving your seats to go to the lavatory, etc. The seat belt is not restrictive and not something uncomfortable. If people can wear the seat belts in their cars without problems, why they need to unbuckle right after that seat belt light turns off? I even keep my seat belt fastened when I sleep in lie-flat business class seats.
Small problem with that. Far too many people don't wear their seatbelts in cars. The US has quite the market for devices that only exist to silence the alarm in the car from an unbuckled driver's belt.
The other main reason for always wearing your belt is that you don't injure others. Some of these injured are likely people who abided by the belt light and were belted up. Only to be hit by someone flying through the cabin.
Yes, it's always a good idea to leave the belt on, even if a smidge loose. If only to protect others. But don't pretend that only idiots have been hurt. There are perfectly valid reasons to not be belted in, the flight attendants will still have duties to attend to unless the pilot in command has made it clear that everyone sits.
Short of requiring everyone to wear nappies and be locked into restraints, you'll always have someone flying through the cabin in these incidents. Until we have a magic wand we instead keep up the communication to people on why they want to stay belted up, expose them to these incidents to get the emotional message across, and improve our sensors and predicitve models to avoid turbulance this nasty.
Not the answers we want in the heat of the moment to punish those who were being stupid. Just directions on how to mitigate the harm of idiots and the simply unlucky.