This is nothing to do with subtle economics. It's extortion because you're in a foreign country and desperate. I think we are just talking about a very unethical company.
With a credit card, they can place a $2,000 hold on your card. If you've only got $100 in your debit account, it makes it a lot harder for them to recoup costs if you damage the car, above all for someone international. The whole point of debit instead of credit is you can't spend more than you have (usually). Car rental is in a pretty unique category of purchase, to be honest, in that they're trusting you temporarily with equipment worth tens of thousands of dollars.
(Debit card holds do exist sometimes, but can be much more limited than credit card holds, and may have additional international restrictions.)
I've never heard of charging extra to allow it though. I guess they ran they numbers and calculated the actual additional risk they're exposed to?
There are credit cards out there (e.g. student ones) with abysmally-low limits. And I can easily keep a hold of ~$2,000 on my Visa/Mastercard debit card, as long as I have that amount in my bank account.
They placed a $1 hold on the card. If memory serves, I've also done this with Hertz but to the tune of a $200 hold.