Why did they pursue D.B. Cooper?

The FBI will no longer look for D.B. Cooper.

In Cooper's 1971 hijacking of a Boeing 727,  no one was hurt, passengers weren't traumatized because they weren't even made aware the plane was hijacked, and the sum stolen was only $200,000.

That raises the question: Why did the FBI spend so much time and money pursuing him in the first place? 

The subject can quickly turn, in online comments sections, to general FBI incompetence. And its ethics, considering Director James Comey's recommendation against pressing charges against Hillary Clinton.

I think that misses the point.

Normally, I oppose the FBI. It shouldn't exist.

But of its few Constitutionally-legitimate functions, investigating a hijacking-for-ransom on an interstate flight is one.

And if this investigation shouldn't have been aggressively pursued, why should any?

If we let the criminal get away with it, wouldn't he be encouraged to commit the same crime again? Or use his criminal profits to fund other criminal schemes?

The purpose of capturing criminals and removing them from society is so they won't commit crimes again.

And as commenter Steven Sizemore notes:

Even so, there's a lot of value in the FBI's deep investigation; to deter others if nothing else. Deterrent came from the public knowing the FBI was going to look under every stone, and keep looking. Maybe they came up empty on this one, but I warrant the high profile investigation deterred other attempts all the same!

Most of the laws we live under are unjust and unnecessary. But there is some comfort for me in knowing that if you commit a real crime, like issue bomb threat in order to steal, they will come after you.

Even if they don't get you, you pay a price. As I've noted on social media:

Q: If he survived, what's the only thing worse than being D.B. Cooper and not being able to tell anyone?
A: Prison.
Was it worth it?