Amber and emergency notices are optional on AT&T too. It’s basically a system-wide text, I see no reason why GSM or CSDA makes a difference.
In the unlikely event that a Presidential alert is issued (by Obama or whomever succeeds him), you’ll get the message just like any AT&T subscriber will. It’s an FCC requirement.
The GSM system automatically pipes the notices to phones that are equipped for them. CDMA isn’t designeds to do this, and requires the system operator to direct the notices for them to go through.