It was considered unethical when the US was founded, but the role and ethics of public prosecutors have evolved in the past 200 years. Also worth pointing out

It was considered unethical when the US was founded, but the role and ethics of public prosecutors have evolved in the past 200 years. Also worth pointing out that local prosecutors are elected, so there are tons of perverted incentives in trying cases. When prosecutions get backlogged, voters get frustrated and will vote for change.