These are government-issued safety messages that include AMBER Alerts, emergency alerts – such as man-made or natural disasters, and Presidential alerts.
Wireless Emergency Alerts are part of the FCC’s CMAS program and are mandated by law. You may turn off alerts (except for Presidential alerts) if you choose. Go to Settings >Notifications>Turn On/Off.
When the software update is delivered to your phone, you will see a message that states “Carrier Settings Update: new settings required for your device have been installed.” After that, your iPhone 5 or 4S will receive all Wireless Emergency Alerts by default.
While the software update is mandatory, you are not charged for the data to deliver it and Wireless Emergency Alerts will never count against your messaging plan. The update will be sent to users with iOS 6.1 or higher. Customers on eligible hardware but not on iOS6.1.x will not receive the carrier update over-the-air. When they next update their software via iTunes, the update will include the current carrier settings.
Source: AT&T Consumer Blog
T-Mobile surprises customers with U.S. Presidential Emergency Alert ahead of Nov. 9 test – November 4, 2011
U.S. gov’t mandates special chip in all cellphones; users can’t opt out of presidential messages – May 11, 2011