Glad the info is helpful. I think the system has good potential as another way to get alerts out there if people miss other, traditional channels. Although, if the lines to your local tower are taken out by a tornado, I would hope the alerts go out long before that!
To answer your question, first you have to know about the types of messages that can be sent. The spec uses 30 total message identifiers, which fall into groups such as Presidential, Imminent-Extreme, Imminent-Severe, AMBER, Test, and Reserved. These 30 identifiers are consolidated into these groups before being presented to the user. This is a bit of a simplification, but you can probably get the general idea. The specification says that by default, phones will be subscribed to all alerts, but that they can opt-out of everything except presidential alerts. So maybe this is a small step towards Big Brother, but at least it is one way only! :)