I think its important to understand that you're never found innocent; only not guilty. The difference here is that you're not guilty given the evidence and arguments presented to the court vs you've been proven innocent.
Secondly, the prison system in the US is meant to be one of vengeance and a continuation of slavery as clearly stated in the 13th amendment[1] rather than one of rehabilitation:
"Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction."
Secondly, the prison system in the US is meant to be one of vengeance and a continuation of slavery as clearly stated in the 13th amendment[1] rather than one of rehabilitation:
"Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction."
[1] https://constitution.congress.gov/constitution/amendment-13/