Under what circumstance can a bus carry a small amount of hazardous material that isn't on the automatically allowed list (ammunition, hospital supplies, drugs)?

AAMVA CDL Manual, Section 4.2.2, p. 4-2

This exception exists for genuine necessity, such as a small town where a bus is the only transportation link available, not convenience. A driver who understands the exception's narrow scope is less likely to be talked into accepting hazmat that should go by another carrier.