"Your" modem is netbooting "their" firmware and they have full remote access on a management interface.
Plus depending on model (like Arris modems), I can do things like set the password of the day seed (away from the factory default) to further lock it down and gain management access remotely.
DOCSIS is slowly dying here anyway and bleeding customers because the cable providers are not competitive when it comes to internet. If they didn't have better linear TV packages the bleed would even be larger.
For many modems on the customer market this also can mean that the ISP can push their own version of the firmware for a modem if you buy identical - such as pushing SURFboard updates.