Seems you got pretty much every single detail of this wrong.
> You used Upwork as your credit card processor.
No, he used Upwork as his agent for selling freelance work. Upwork in turn used whatever credit card processor they used; perhaps their bank, I don't know.
> Then the processor told you that your customer was basically using a stolen credit card.
No: Then Upwork's credit card processor told Upwork that Upwork's customer was basically using a stolen credit card.
> It’s completely reasonable for Upwork to take these funds back from you.
No: It’s completely reasonable for Upwork to take these funds back from Upwork's customer, "Robin".
> And it’s completely reasonable for you to call “Robin” and demand he pay you back directly.
> You used Upwork as your credit card processor.
No, he used Upwork as his agent for selling freelance work. Upwork in turn used whatever credit card processor they used; perhaps their bank, I don't know.
> Then the processor told you that your customer was basically using a stolen credit card.
No: Then Upwork's credit card processor told Upwork that Upwork's customer was basically using a stolen credit card.
> It’s completely reasonable for Upwork to take these funds back from you.
No: It’s completely reasonable for Upwork to take these funds back from Upwork's customer, "Robin".
> And it’s completely reasonable for you to call “Robin” and demand he pay you back directly.
No.