That's why I'm not an AT&T customer. Spectrum lets me bring my own hardware, and they're the only other option in my area, so Spectrum gets my business. Plain and simple. Unfortunately, not everyone has the palatable solution that I have.
AFAIK, they can/will kick you off the network if your modem is running unverified firmware. I think this is a regulatory requirement, but don't take my word for it. They don't want anyone to have free access to the network, you could do things like spoof your MAC address to get free service. I'm sure you could also do something much more malicious like crash parts of the network.