> I suspect that the idea originally was to give some safety valve

That reminds me of the early 2000s, where there were a lot of US debates around around terrorism and "harsh interrogations" i.e. torture.

A certain bloc of politicians and commentators kept bringing up a hypothetical scenario where there was a nuclear bomb counting down, and some guy wouldn't admit where it was hidden in a major city. My favorite response to that involved presidential pardons, something along the lines of:

1. "So what? If everything you say is true, then the authorities would simply torture the guy and seek a pardon afterwards. We already have an exceptional mechanism for those exceptional situations, meaning that's not a reason to change it."

2. "Conversely, any interrogator who isn't confident of a pardon is on who does not believe it's at ticking-bomb situation, meaning they cannot justify torturing someone anyway, they just want to do it to make their job marginally easier. That's bad, so it should stay illegal."