Maybe this is a bad analogy, but it’s the only argument that makes sense to me.
If someone gives you something as payment, no strings attached and no misrepresentation, then it's a fair play. They gave you a watch as payment, they didn't ask you to sell it and pay back the difference. The debt is settled even if the watch isn't worth enough, because it was the payment, whatever its value ends up being.
If you took the watch to repay a debt, then yeah, it's unethical not to pay back the difference. Thinking about it in the converse helps: if you took the watch and it wasn't worth enough to pay back the debt, you would probably insist they're still on the hook for the rest. Cuts both ways.
I think for property tax cases like this, once you forfeit the property the debts are cancelled.
It depends on whether you knew that what you were buying was worth more and whether you bought/got it with intent to deceive the other person about its true value (which is a hard thing to determine in the moment)
Wasn't it clear in the article that the issue was not whether the county was justified recouping the unpaid taxes, interest and penalties, but whether it could keep substantial gains above and beyond the amount of the unpaid taxes, interest and penalties?
Wouldn't this be a debtors prison?