Why do they throw people in prison for murder if it was an accident?
Persons are not convicted of murder if the death was accidental. Proving that a murder was committed must include proof that the perpetrator intended for the victim to die or that the perpetrator should have reasonably expected that his actions would result in the death or that the perpetrator acted with reckless indifference while committing a dangerous act. They could have been convicted of vehicular homicide (in which no intent has to be proven) or manslaughter (in which harm, but not death, was intended) or some other homicide. Check the offense in the case you are talking about. It is unlikely to have been murder, though it may have been another form of homicide.