Re: How to enforce
"And how exactly are ISPs expected to get this info?"
They don't. As I understand it, Big Media <TM> tracks the IP address of the culprit down, the ISP is forced under legislation to match that up to a user in their database, and again up to the ISP to send the letters.
In other words, Big Media <TM> is making the ISPs do their dirty work for them.