I think it is just a bad habit parents got into when the kids were little they can't/won't break now. Almost all of my kids are picked up and the first thing they ask for is juice and mom says yes your juice is in the car. The kid has had a full snack less than an hour before and will be having supper within the hour after pickup so there is no reason for needing another one but it has always been that they get a drink of juice and a container of dry cereal or granola bar or those fruit bites when they are in their car seat to keep them quiet. Like the kid can't sit quietly for the three block ride home. I can see treats on an 8 hour drive somewhere but we are talking minutes.