I'd be interesting if those passengers injured by other passengers not wearing their belts file law suits against them for negligence. If successful, this may then encourage more people to fasten their belts when seated.
As the cause of being thrown about is the aircraft, not simply being on the flight, it would not be worth bring the case. Additionally you never attempt to sue a injured party like this, your on the back foot from the beginning (establishing reasonable, foreseeable and standard practice).