Yeah, I think it's a Des Moines metro (maybe more of central Iowa) and actually also a St. Louis thing.
I actually think it's a fun tradition and wish more places had it.
Yeah, when I first moved to Illinois, right after college, I was pissed my first Halloween. I asked my wife, why do all these lazy kids come up and just say Trick or Treat? She asked me what I expected them to say. (she's from Illinois)
I'm more used to it now, but considering I've always given out full size candy bars, that I should at least get a little joke or at least some thoughtful words. Especially since, when we first moved into our neighborhood in '97, we were known for full size bars and were a lot of kids' first stop.
Maybe I'll have both full size and king size next year. tell them, "Trick or treat," gets you your pick of full size bar, tell me a good joke and it gets you a king size bar.