So why on earth doesn't Harnett then?

I thought your argument was that Harnett's lenders are equally to blame for any problems they have experienced due to his default because they should have done more due diligence before lending to him.

Surely that would apply in this case too?

In my opinion, a person who borrows a handful of coins and then can't pay them back due to unforeseen circumstances is much more deserving of trust, compassion and leniency than someone who starts up a commercial enterprise that is based on borrowing coins that goes into default based on poor investments

It's Harnett that should be called to account for his lack of due diligence. He was in the business of lending out other people's money for his own profit, and therefore should be held to a higher standard.