Would We Execute Convicted Criminals This Way?

Let me make it clear that as a pro-lifer, I oppose the death penalty as much as I oppose abortion, euthanasia, embryonic stem cell research, artificial contraception, and ALL forms of sexual immorality. However, if the court imposed death sentence for Terri is such a "peaceful" way to die, why does society prevent its use for criminals who are executed? Could it be that if such a method was used, criminal rights groups would rightfully be up in arms about it? (Sadly, I notice they are strangely SILENT about Terri Schiavo's execution. Evidently, convicted criminals have more rights than Terri Schiavo.) But really, for those who have no problem with Terri being killed in this manner, explain why it is not used for convicted criminals.