It doesn't matter if state prisons violate human rights (and many do), because Assange isn't going to a state prison. He's going to a federal prison, and those facilities are much nicer, and their staff better trained. They're also monitored up the wazoo by lawyers for rights violations.
Assange's team would need to show that the prison he would be assigned to regularly and currently violates human rights, and he simply can't do that.
Assange's team would need to show that the prison he would be assigned to regularly and currently violates human rights, and he simply can't do that.