I recognise the figure of 70% recidivism. It's the same here in the UK. For all I know it's the same everywhere.
The one thing we DO know is that prison doesn't work. (If we think that it's some kind of solution to re-offending that is.) The one and pretty much only thing we can say on the positive side for prison is that it prevents further crime in the community - but it doesn't prevent crime in the prisons themselves.
The sad fact is that the prison population isn't made up of murderers and rapists, it's mostly full of the illiterate, mentally ill and drug addicted. It isn't a deterrent because those that end up there don't have the mental capacity to imagine being caught or don't care if they are.
The truth is, we just don't know how else to deal with the problem.
As an aside and as an outsider, looking at how the US deals with its criminals seems mediaeval which sadly we seem to be following..