Is there any evidence that criminals conduct more criminal activity during their prison sentence than the general populace? If there isn't any, then either you should be monitoring all citizen calls, or get rid of this arbitrary punishment for criminals. If there is such evidence, is the difference substantial enough to warrant the surveillance?
>Is there any evidence that criminals conduct more criminal activity during their prison sentence than the general populace?
The point is rather moot, considering that one of the main purposes of incarceration is incapacitation (effect of a sentence in positively preventing, rather than merely deterring, future offending).
I.e. you cannot steal, rob or kill because being monitored in prison prevents you from doing so. Lack of evidence therefore wouldn't make much of a justification for removing such monitoring because it could be argued just to show that the monitoring works.
However, without digging into statistics I would expect that there is more crime in prisons than outside (not because incapacitation wouldn't work at all, but because offenders are concentrated in prison).
There is substantial violence in prisons between inmates. (Not only in the US but everywhere).
