The fussy litterbug

I was riding the subway the other day, sitting across from a woman and her child, who looked to be around ten years old. They were eating something, not sure what. When the train pulled into the station I watched her unapologetically toss their garbage in the bag outside the door and onto the station platform. I couldn't believe it. I looked right at her. She saw me and just looked away, but not in any way that conveyed she felt badly about what she had done. Then, a few stops later her kid was halfway through a lollipop that he decided he was finished with and she told him to wait until the doors open and toss it out. He got up, waited for the doors to open, and aimed the lollipop right through the gap and dropped it onto the tracks. She praised him for making the shot. I was absolutely amazed.

Now, I get why people litter; they are lazy, they don't care, they don't respect shared space or the city they live in, they have no sense of human decency, etc, etc. But what I don't understand is this: How did she come to the conclusion that it is wrong to litter on the train, but perfectly acceptable on the platform? I continue to be baffled.