The bus driver is driving. And a multi-ton bus. It is dangerous for him to get involved in conversations between passengers. A bus driver is not liable for their conversations.
I agree. However, if there is physical force implied or applied, the driver is responsible to pull over and call the cops, and be a witness when the offender is thrown in jail.
I really don’t see the issue in most cases anyway. Who goes from Will to Boro Park except Frum Yidden?