Because it’s a prison. Voice monitoring is disclosed when calling from a prison. Public or private makes no difference.

The purpose of a prison is to hold those who have lost their rights because they have violated the rights of others. To fulfill their purpose, they must maintain a high level of security, both to prevent escape and to prevent further harm to the public. Pervasive and invasive surveillance of the prison population and facilities is a necessary consequence of their legitimate purpose.

Private vs public has no implications here. Both house people who broke laws. You aren't being voice printed in your home by the American prison system be it public or private because you aren't in the American Prison system.

I'm not really following your logic here...

We should consider changing/limiting that.