It doesn't really matter under what setting it's placed in the OS, but "emergency alerts" or CB is clearly network setting/service, so it's not surprising carrier can control status of this setting/service and override what you set in the phone.
Not... really? It's not the setting for whether the phone receives alerts; it's the setting for what the UI does in response (make a bunch of noise vs accept silently). The UI is very much on the user-controlled side of the phone.
they try to avoid people accidentally switching it off, it's much less harm if you hear it than vice versa
I'm aware that this is their reasoning. I do not accept this as an argument for it being a carrier setting.