The judicial system is extremely unreliable-- this in itself suggests that denying cryonics as though you're sure that all the prisoners with life or death sentences are guilty isn't sound.

This is a good point. Perhaps the courts could specify a minimum timeframe to keep the person cryopreserved, which is longer if the evidence is weak. At the end if the criminal is still considered guilty they are thawed and buried. If they are exonerated, their preservation becomes indefinite until a resuscitation method is developed.