What if you refuse to fasten your seatbelt?
In the US it is an offence under federal law.
A loose 180lb/80kg projectile inside an aircraft is a threat to other passengers during turbulence, in the worst cases of passenger misbehaviour cabin staff can and have forcibly restrained passengers and/or diverted to a nearby airport to have the passenger deplaned and sometimes arrested.
Seven Ways to Get Kicked Off a Plane
Family kicked off plane

A 3-year-old, who refused to use his safety belt, caused a flight to be delayed after his father could not get him locked in his seat belt on board an Alaska Airlines plane.
The pilot returned to the gate on Saturday and asked the father and child to get off the plane.

Why you need to wear a seatbelt

For passengers who refuse to wear seatbelts when the sign is illuminated and who are disruptive to the crew, the FAA can impose a maximum fine of $25,000.