In reality there may not be enough time for the ISPs to respond to each act of piracy before a new one is commited.
And in the end, all they will do is force the pirates down into the darker corners of the internet. A good thing for freenet (better that people trade the latest Game Of Thrones episode than the latest 40 year old rapes children and films it for your enjoyment), but a bad thing for the internet, and society at large.