The FDA responded.

In their form letter response, they indicate that only scheduled drugs must be declared.

I don't know where one might find data, but that's still a significant number of people crossing through customs. In fact, a lot of people take a benzodiazepine (Ambien, Xanax, etc.) only when they fly.

In fact, the majority of prescriptions I write for these drugs are for flying only.

So there are two problems:
1. A discrepancy between what the FDA says and what CBP claims to enforce
2. Still a questionable need for passengers to stop and declare their Ambien

In fact, the FDA says that this includes passengers leaving the United States. If one were to follow the law, this would mean asking the airline to call a Customs officer to come to the ticket counter or gate to say, "I'm traveling with five Ambien, and the law says I need to declare them to you."