A lot of it is the sheer bureaucracy of chasing down actual pirates and weeding them from people who just happen to be on the same IP address.
If one guy visiting an apartment block downloads a torrent from a public connection, what is ATT supposed to do? Shut down Internet to the entire building?
This is an undue burden for ISPs, even if the content isn't living in a gray zone of legality.
Yeah IP owners really want to have all the benefits of ownership with none of the drawbacks. After lobbying for and receiving a blank check to be able to rent seek indefinitely, they are constantly acting to outsource any cost of detection and enforcement of "their" property. Disgusting how goddamn entitled they are.