It would correlate strongly with people trying to evade taxes and/or launder money. PayPal doesn't like to facilitate illegal activity. I'm not theorizing about this -- I have first hand knowledge.
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.