The Federal Motor Carrier Safety Administration established these limits to make sure that these drivers are always safe behind the wheel. There are reasons for this; for example, if a bus driver drinks and has a BAC of 0.09, the damage that driver could do while drunk is significantly higher than a normal driver of a passenger vehicle.
A bus driver more than likely will have dozens of people on board, and the bus itself is much heavier and likely to cause heavy damage when it crashes. Without seat belts, those on the bus could be thrown or killed; on top of that, passenger vehicles that are impacted are more likely to be crushed or severely damaged.
Drivers are also not allowed to drive if they’ve had alcohol within the last four hours. This makes it even less likely that a driver will get behind the wheel when drunk, even if they had been drinking that day. Instead of risking a driver drinking and testing below 0.04 only to have the alcohol content of the blood rise after he or she begins working, the worker will have to wait four hours following the last drink, giving his or her body time to regulate the amount of alcohol in his or her system.
Source: FindLaw, “Commercial DUI Regulations,” accessed June 02, 2016