It makes complete sense to me that given a spec of "make a button send this specific text" you would end up with this. So it's very easy to send the initial message but sending a custom follow up requires contacting someone with access to the underlying system who can send a custom message.
In the same way that I may give internal users the ability to fire off SPECIFIC (maybe with placeholders) push notifications with the press of a button but not custom messages. Sure my underlying infrastructure can handle custom messages but the UI for such a feature does not exist (or at least is not available to the person who might have the ability to send preset messages).
All in all this was terrible for the people in Hawaii, no doubt. That said I WANT my missile alert system to be easy to activate to give people the most time I can to prepare. The best fix for this (yes a new UI/UX would be best but let's be realistic on how much time/energy they are willing to spend on this) would be a scary looking alert confirmation dialog of "Are you sure you want to send this alert to the whole state?" and maybe have to type a confirmation string.