When using Paypal WPS, no account is created for the anonymous user after the payment is completed. The user information recieved from the IPN are never stored in the order.
To fix this, I've added the following code in uc_paypal_ipn.
$order->primary_email = $payer_email;
Having an e-mail address enables uc_cart_complete_sale_account to create an account for the user.
Now I'm wondering if there are any side effects to this hack. And even more if nobody else noticed this issue.