Likely somebody made a payment with a stolen credit card.

And why would that be the problem of the receiving end of the transaction?

Money laundering

But this could happen to any business, and surely does. Completely turning off a business because it receives one fraudulent payment is not appropriate. Indeed, this hasn't been my experience with other billing platforms.

Of course, we're taking the posters claim that this is what happened at face value...

If it's your first payment, you're pretty much done for. One of their targets is any operation where an entire business/yt account/paypal account is made to launder a single transaction and then left to rot.