When was this? I’ve been using Comcast for five years with my own modem. Comcast’s own web site has a list of third-party modems that they officially support, which you can buy and use instead of the one they lease.
If anyone is telling you that you have to rent a modem, they are flat-out lying to you.
And this has always been the case. The DOCSIS standard is just that - a standard. And one that Comcast participates in the development of. They can use literally any DOCSIS compatible modem as long as their server has its MAC address (and it supports the bandwidth you’re paying for). They have a self-installation system that usually works to register your modem with them or you can call customer support to give them the MAC address for them to enter.
In the case of a cable modem, only the cable company can upgrade the modem’s firmware. I don’t know if it’s due to some law, but no cable modem (or router with built-in modem) gives you the ability to install your own firmware (at least not without lots of hacking). Instead, the cable company needs to push firmware updates to you.
This is one of the reasons why I never recommend combined modem/router devices. As a combined device, you’re at the mercy of your cable company for firmware updates. As separate devices, you can upgrade the router on your own, even if you have to rely on the cable company to upgrade the modem.