Clarification: is this snark or flamebait? I might be missing something, but it seems like a reasonable question. I recall the first time I heard she'd done that, my own incredulity was similar (having never been in a life-or-death crisis, I didn't have the frame-of-reference to get that a person may not be thinking rationally "Doctors can't put brain parts back in" and might grab whatever they see come off a loved one).
I suspect the minimized version, "Why would she do that?" would have been well received. I don't think anyone understands why the "I'm not from the US" clause is there and I personally assigned a reasonable probability that it was a short-hand stand-in for the following sentiment :
"The US has so much gun violence that they must have some kind of expertise that leads them to know that its a good idea to save skull fragments in case the doctors need them. Lol, just kidding, that was sarcasm; I just wanted to bash the US for their high rate of gun violence."
Of course I accept that this may be mis-interpreting it. The only other meaning I can parse is: "I imagine that everyone in the USA learned about this event in great detail in their school lessons/etc. I haven't so....could someone explain why she grabbed the skull fragment?"
She reached for it within the fraction of a second that the bullet struck JFK's head. It was an instinctive action.
In fact, we humans usually can't rationally comprehend catastrophic injury to the body, while in that state of immediate shock. Think of the image of the soldier carrying his detached arm.
She was in shock, and tried to do something to help her husband. It's not even that far fetched. If your hand was cut off, you'd want to save it for possible reattachment.
It's really impossible to speculate until you've been in a situation like that and most of us probably have not. I would suspect it was a panic response and not some deeper nefarious action.
Sorry, not from the US but why would she do that?