Could be a regional thing.
I know around here (Eastern Ontario), you are definitely supposed to. You're also supposed to flush your dog poop (the city requests it since their facilities aren't set up to process human or animal waste), but based on the many little baggies filling up public cans no one actually listens to that particular advice.
Flushing baby poo, on the other hand, actually seems to have stuck, maybe because it makes sense to people in a way that flushing dog poo doesn't. They enforce it at daycares, drop ins, and I was taught to do it as a babysitter. Just good manners, really. No one wants to deal with a lot of poop in the garbage. Especially not in your own home. It's worse than rotten fish, and you can't exactly stick it in your freezer, either. Nor would your neighbours thank you if you put it outside next to their driveway.
And when my kiddos were two, it was useful when we started toilet training. "Where does the poo go? In the toilet!" It was natural to go from shaking it in from the diaper, to going on a training seat and tipping it in, to actually sitting on the toilet and going.