How about this? Suppose someone heard a gunshot in, oh, I dunno, the Washington D.C. area. He goes out and finds a spent case from a .223 rifle and surmises that it was evidence from a serial killer who has been terrorizing the city. This case could be instrumental in linking the victim to a gun to a killer. If he decides to keep it so he could sell it later on eBay – “Actual bullet case used by D.C. Sniper!” – he is imeding an investigation. If his actions resulted in a delay in catching the killers and the killers are allowed to kill for another day, another week, another month… then his taking of a piece of evidence for a souvenier or for profit is costing lives.
Is Farmer Bob a rocket scientist? Is he a crash scene investigator? Is he a metallurgist or aerospace engineer? Probably not. Certainly most pieces of a crash, of Columbia or a passenger plane or whatever, are not critical to an investigation. But how do laymen know which pieces are? Not talking about Columbia specifically, but for illustration: Suppose someone finds a sheared rivet. No other rivets from the same area are found, for whatever reason. Why did it shear? Is there a flaw in it? Were all of the rivets in the structure where this rivet came from defective? Who would even know to find out, if a souvenier hunter takes the onely one to be discovered? Until a piece of debris is ruled out as a critical piece, then it is de facto a critical piece.
Put another way:
For want of a nail the shoe was lost.
For want of a shoe the horse was lost.
For want of a horse the rider was lost.
For want of a rider the battle was lost.
For want of a battle the kingdom was lost.
And all for the want of a horseshoe nail.
A nail found in a field is a worthless thing. But how would we know that the kingdom was lost becuase of it, were it not for finding the nail? How will we know what caused the loss of Columbia if someone steals a piece of seemingly-worthless debris that by coincidence happens to be a key clue?