One also has to remember AF1 is not a fore hire flight, so most rules governing normal passenger flights do not apply to a flight flying the president. Sure seat belts are part of the basic rules but that also applies to private jets and nobody is really there to enforce those rules nor is needed.
Truthfully most of those 'seat belts while seated' rules are there to protected the airline from liability when you hurt yourself and try to come after them for damages. It's only rarely (statistically speaking) that the seat belts are truly protecting you from what they were designed to.