While I do think it's poor etiquette, you can pretty easily play devil's advocate and see it this way:

Your were coming down a descent a FAST speed, nearing 30mph. While you (and many on these forums) will have confidence to take a hand off and signal at that speed, these guys may not have. I for sure have bombed down windy roads (fortunately not with anybody behind me) at similar speed, encountered an unexpected branch or pebble, and not had time or confidence to take a hand off the brakes to signal it.

I'd actually say it was far more likely that they weren't out to "screw you" , but that they were trying to ride safely within their limits at that type of speed.

I already know you're going to say "but it was totally clear" and "totally safe to signal with no curves, turns, or braking areas", and that'll probably be true to you, but you already knew that you can't assume everyone out there is as good a rider as yourself.

I actually have found that that are very few real strangers out there who are out to be 'jerks' , but many, many more instances of incidents where one person misinterprets the action and immediately concludes that they were intentionally wronged by a 'jerk.' I don't know any cyclists, even bad ones, who would split quickly just to wipe out a rider behind them, and I've been taken down in a similar fashion in a fast ride splitting around unexpected traffic cones with no signal warnings despite at least 7 riders ahead who were too busy emergency braking and swerving to signal until it was too late. (Good riders, too - a few Cat2-3s in there)