Apparently, the Emergency Broadcast System is like a phone tree. The annoying two-tone signal serves to get your attention, and also any other EBS stations listening for an EBS broadcast. They then rebroadcast the message.
Of course, phone trees are not a good way to run a national information infrastructure, so in 1998, we moved to the Emergency Alert System, which gave Mr. President the power to instantaneously interrupt your rerun of Falcon Crest. Unfortunately, the FCC isn't very informative when it comes to describing EAS.