That's certainly possible, but I'd be surprised if it came to pass. Anyone who stopped by your house and used your wifi would need to take on your ISP keys, you

That's certainly possible, but I'd be surprised if it came to pass. Anyone who stopped by your house and used your wifi would need to take on your ISP keys, your DVD player would need to be able to accept them, etc. At this point the ad revenue wouldn't seem to make up for the bad user experience. It would be similar to an ISP requiring you to trust their root CA so they could MITM your SSL sessions. Sure, some enterprises do this through their management systems, but a big ISP that required this would likely get hit by a large amount of negative reactions.