Ask HN: Why does the emergency alert system require OS updates from carriers?

Is there a network-related reason that carriers such as AT&T and Verizon needed to (preemtively) update iOS and other smartphone operating systems in order to be able to issue emergency alerts (i.e. amber alerts, weather alerts, and presidential alerts)?

It seems to me that alerts can be sent as SMS messages and they could have simply added an option to their online account management system that would allow suscribers to opt out of receiving the alerts (except for presidential alerts apparently).