I don't understand why. This isn't how it works with other systems. If I get a ton of chargebacks on Paypal, they'll try to take the money back out of my bank account. And if they can't they'll grab it later the next time someone pays me (what Upwork is doing here). And if even that won't work, they'll just close my account. There's no scenario where PayPal is expected to continue working with me, because I'm now a fraud risk in their eyes.
Ditto on Amazon: get a bunch of returns and you'll need to make that up or be cancelled.
People are not really understanding the situation here. The OP's complaint isn't that Upwork is suing him or otherwise attempting to force payment. It's just that Upwork won't work with him anymore unless they get their losses covered.
And... maybe that's not fair, but that's how life works on the internet of payments. Upwork's behavior is very typical here, and no one should be surprised.