Well if they actually decide to sue the customer, and get a court order, they could force the ISP to identify them. But that's way more trouble than it's worth

Well if they actually decide to sue the customer, and get a court order, they could force the ISP to identify them. But that's way more trouble than it's worth in most cases. And safe harbor laws mean that as long as the ISP is adhering to the notice-and-notice policies, they can't be directly sued for infringement.