The ONLY time it's considered acceptable to use physical force is in physical self defence. This is a fact when it comes to legality, and also my strong opinion when it comes to morality. The latter is up for debate but the former I'm afraid is just not; putting your hands on someone for calling you names is assault. It's not a grey area. Whether or not someone is CHARGED with assault for it would depend on if injury was caused, provocation would be taken into account, and so would past conduct. But it's still assault. The bus driver would most likely only be warned not to do it in this case - if he's previously clean. If he makes a habit of it hell end up being charged. Because it isn't legal.
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