The Federal Motor Carrier Safety Administration requires that commercial drivers to wear seat belts. See 49 CFR 392.16:
(a) Drivers. No driver shall operate a commercial motor vehicle, and a motor carrier shall not require or permit a driver to operate a commercial motor vehicle, that has a seat belt assembly installed at the driver's seat unless the driver is properly restrained by the seat belt assembly.
(b) Passengers. No driver shall operate a property-carrying commercial motor vehicle, and a motor carrier shall not require or permit a driver to operate a property-carrying commercial motor vehicle, that has seat belt assemblies installed at the seats for other occupants of the vehicle unless all other occupants are properly restrained by such seat belt assemblies.
You will note that only passengers in a "property-carrying commercial motor vehicle" are required to wear a seat belt. The passengers in a commercial motor vehicle that is designed to carry passengers and not property - think tour bus - are not required to wear seatbelts. I'm unclear what the logic here is, but nevertheless, this is what we have to work with.
Consequences of violating 49 CFT 392.16
A violation of 49 CRV 392.16 is a criminal charge according to A.R.S. 28-5240, which states:
A. In addition to civil penalties imposed under this chapter, a motor carrier, shipper or manufacturer who operates or causes to be operated a commercial motor vehicle in violation of this chapter or who knowingly violates or knowingly fails to comply with any provision of this chapter or with any rule adopted pursuant to this chapter is guilty of:
1. A class 2 misdemeanor for a first offense.
2. A class 1 misdemeanor for a second offense.
3. A class 6 felony for any subsequent offense.
For a first offense, which would be charged as a class 2 misdemeanor, there is the potential for up to a $750 fine plus surcharges of $630, up to 4 months in jail, and probation. In practice, most drivers will face a fine and the misdemeanor conviction; jail time and probation are unlikely.
A big problem with these cases is the opportunity for the police officer to observe the driver of a commercial vehicle while everyone is driving. Did the police officer actually see the driver driving without a seat belt? Did the driver remove his seat belt after he had stopped but before the cop came to the window? A similar problem exists with the using a hand held mobile device and texting while driving violations.