Really! This is the first I've heard of this one, it's a good thing I have that bell on my bike.
I have encountered horses being ridden on bike routes/MUPs here, and the riders do appreciate it when the cyclists call out a warning and slow down before passing. I never want to spook a horse.

Back on topic: by following the laws, traffic is more predictable, less chaotic, and presumably safer and less stressful for all. The "everyone else does it" argument belongs to gradeschoolers.