Why can't prison inmates be used as medical test subjects?

Because, even though they screwed up and landed in jail doesn't mean they're not deserving of compassion and proper treatment. Also people are wrongfully convicted more often than you'd realize.

The big thing everyone needs to remember is:

TREAT OTHERS HOW YOU WOULD LIKE TO BE TREATED.

and

PUT YOURSELF IN THEIR SHOES.

...morality issues, ethical issues, goes against their human rights, undermines the justice system, punishment after punishment, possibility of death, family objection, possibility of bodily harm, maiming, sensory/limb loss etc etc etc.

Without his consent? It could be taken as ' cruel and unusual punishment" which is illegal.

They can.

WITH THEIR PERMISSION.