Bags on seats

I notice there's a lot of grumbling going on in the blogging world (including mine)so allow me to add yet one more whinge: what possesses commuters to publicly display their ignorance by putting their bags on the seat next to them? It wouldn't be an issue on a train in the middle of the day when there's plenty of seats but passengers do it in peak hour.

I shouldn't have to ask someone to move stuff off the seat when it's not supposed to be there in the first place. I suspect that they just don't want people sitting next to them but that doesn't work when the train is crowded. Other possibilities are that they have a loved one or friend getting on at a later stop and they are reserving the seat for them (I was actually scowled at once because I wouldn't move from my window seat so a couple could sit together) but this can't apply to half a carriage of people on a limited stops train. Or maybe they view it as a way to start a conversation but it's doubtful that their intended communicee would be kindly disposed toward them given their apparent lack of consideration.