According to Idaho Statutes ...
What types of deaths are required to be investigated?
(1) When a county coroner is informed that a person has died, the county coroner shall investigate that death if:
(a) The death occurred as a result of violence, whether apparently by homicide, suicide or by accident;
(b) The death occurred under suspicious or unknown circumstances; or
(c) The death is of a stillborn child or any child if there is a reasonable articulable suspicion to believe that the death occurred without a known medical disease to account for the stillbirth or child’s death.
(2) If a death occurs that is not attended by a physician and the cause of death cannot be certified by a physician, the coroner must refer the investigation of the death to the sheriff of the county or the chief of police of the city in which the incident causing the death occurred or, if such county or city is unknown, to the sheriff or chief of police of the county or city where the body was found. The investigation shall be the responsibility of the sheriff or chief of police . . .
Idaho Code Ann. § 19-4301.
What types of deaths are required to be autopsied?
The coroner may, in the performance of his duties under this chapter, summon a person authorized to practice medicine and surgery in the state of Idaho to inspect the body and give a professional opinion as to the cause of death. The coroner or the prosecuting attorney may order an autopsy performed if it is deemed necessary accurately and scientifically to determine the cause of death. Idaho Code Ann. § 19-4301B.
So, one of two things happened. Either a physician determined the cause as a natural death or the Sheriff of Fremont County determined that, at the time, the death was not suspicious.