This is a wonderful question. You are correct that there are some situations in English in which the driver's action would be emphasized in the sentence. If a driver purposely hit a child with a car, most people would not use the passive voice. People would say, "That driver hit that child." However, if the speaker has no idea what the intentions of the driver were, the speaker usually focuses on the person s/he does know about. In this context, the passive voice would be far more common. In, for instance, the child's hospital room, when the driver is not present, most of us would expect to hear, "This child was hit by a car." It would sound strange to us if someone said, "A driver hit this child with a car." Using the passive voice in this case is not an act of hiding guilt; it is merely an act of focusing on the person who is present and important in the mind of the speaker. (I'm guessing that Farsi speakers would not talk about the car's driver in this situation, either. Maybe they'd say something like, "This child is badly hurt." This is, of course, mere speculation on my part.)
That said, you are correct that writers do sometimes hide behind the passive voice. A classic example of this is the sentence, "Mistakes were made." In his Political Dictionary, the writer William Safire calls this sentence a "passive-evasive way of acknowledging error while distancing the speaker from responsibility for it." In other words, this sentence is usually used to talk about problems without taking any blame. Most Americans recognize the evasiveness of this kind of speech and get very impatient when we hear it.