I think it is one of those occurrences where, no matter what the letter of the law says, common sense (and above all, the safety of passengers and the driver) should take precedence. That might mean stopping the bus there and then and transferring everybody, or it might mean driving away from an area where the vehicle or its passengers might be vulnerable to further attack. It might also mean not stranding passengers for hours when a replacement vehicle is not easily sourced. Such actions can be seen as reasonable under the circumstances.
In areas where this becomes a frequent occurrence some operators choose to withdraw services for a period, notably in evenings.
I recall reading an article about a coach company local to me, Empress Coaches of Bethnal Green. They used to convey troops in the 1970s when IRA paramilitaries were particularly active, and had a police escort. Now the law of the land requires that if you hit and injure someone whilst driving, you stop and remain at the scene. When on troop movements, Empress drivers were given explicit instructions NOT to stop if they hit anybody.