What percentage of a typical ISPs customer base, download songs? If 19 out of 20 downloads infringe copyright, and each infringement is worthy of an internet disconnection, then almost almost all that percentage would no longer exist as a customer for that ISP.
Once a customer is disconnected, you cannot continue to charge them, so what business can suddenly afford to lose customers in that way? And that is on top of what ever other losses occur from people tightening their belts.