If 1% of all customers are fraudulent but 5% of customers with multi-country bank accounts are fraudulent, that might be enough to tip their risk/reward balance.

And I've dealt with this first hand. I have paypal accounts and legal bank accounts in multiple countries, and when I've tried to move money between them I get a call from customer service saying they blocked the transactions, and "you're not supposed to do that with paypal".

I think you are assuming that paypal operates on reason, but it rarely does.