Edit: dude basically didn't want to wear his seatbelt before takeoff.
At first I think it's a non-issue, let dude fly without his seatbelt. No sense grounding an entire flight unless everyone refusing to wear one.
After thinking about it, it ain't a car. Planes go vertical and if you're sleeping without a seatbelt during take off there's a risk you fall out and collide with someone.
Airline don't want the potential legal problems.
Additionally, as an athlete you're supposed to reduce the risk for potential injury to yourself.