There are laws affecting this at both the state and federal level, but by default, most government records are open for public inspection in the United States. If we can't see what the government is doing and how it's making it's decisions, we can't very well decide how to direct it as voters. 

The extent of openness varies from state to state, but I don't know of any states where an autopsy report -- or a related toxicology report -- would be considered secret. When people are dying of unnatural causes, the people have a right to know about the dangers in their community, whether that disease, drugs or violence.