There are still some PTC sites using PayPal as payment method, one of them is WordLinx which has already been around for 5 years. NeoBux, TheClickers and most of the Aurora PTC sites also use PayPal. But I have never heard of a site using PayPal instant payment. As far as I know, it cannot be done, but I might be wrong.
i think they pay via their own paypal account without being known by paypal.
but if they become a big site then i think paypal will pay attention to them and force them to pay via other currencies like alertpay.