That behavior started at the national level and filtered into local areas with referee programs. Oddly enough, it's those areas with referee programs that would be perfectly fine with referees calling it the way everyone here suggests. But believe it or not, there are entire franchise areas where a double hit foul has never even been heard of. We all know the reaction you get when you call a double hit foul on a player who has never heard of such a thing. The guilty party insists that they never hit the cue ball twice, to the point of becoming belligerent - insisting that they've been cheated. This is not a behavior APA wants in their national events - sometimes it's more important that the participants come away with a positive experience than it is that every situation be treated like it's a tournament for professional players. So when a referee is called to watch for a double hit, they are instructed to tell the shooter what they're watching for and how to avoid it. They're also instructed to inform the opposing team that this is what they're going to do. It gets covered in captain's meetings too, so it shouldn't be a surprise to anyone.
Since the APA chooses referees for their national events from franchises that use referees locally and have referee programs, that practice has found its way into many of those programs as the operators see the benefits of doing so locally and of being consistent with the way it's done nationally. If your referee didn't tell you what they were going to do and it wasn't explained to everyone at a players' meeting before the event, it's understandable that you would be surprised.
Sometimes camaraderie wins out over cutthroat in the APA.