Earlier quoted context omitted.
Can't they use DHCP leases? Thankfully, ISP doesn't show that much effort.
Even if they weren't too lazy to, an IP doesn't and can't identify a person or even household. Must be my neighbors downloading movies on my open wifi network, I can't figure out these high tech router thingymajigs ;).
For example, I asked what happens if you ignore Comcast's Bittorrent warning and someone else on HN said they do indeed terminate your account.
Maybe playing dumb doesn't get you off the hook. Or it reveals you as a dumb helpless open relay. The most I can imagine Comcast doing if you responded to them would be sending over a technician to reset your router password.