If the pilot needs to slam the brakes before the plane is aloft, then the consequences for seat mates could be severe as an untethered pax flies through the cabin.

Or if the plane is taking off in turbulent windy conditions, once aloft, the plane can be suddenly buffeted in any direction.

In the event of an emergency after an aborted takeoff or landing shortly after takeoff, an unconscious, dead, or otherwise immobile pax clogging the aisle or row puts evacuees at risk.

So pax who do not wear seat belts on takeoff add risk to others.