Not sure how this tradition "morphed." In my day, the mids threw candy (as in small, individually wrapped pieces) to kids as we marched. It wasn't a huge thing, the kids loved it, and I'm quite sure THEY (the kids) cleaned up -- as in scooped up all the wrapped candy and ate it. It was a really fun tradition that brought smiles to the mids, the spectators and, especially, the kids.
Now it seems that people are throwing large objects, including bottles AT THE MIDS. First, it makes no sense as they can't eat it while marching and will shortly be at the stadium where they will have plenty of access to all varieties of snacks. I've heard what happens now borders on dangerous and, if it doesn't stop, march overs will be canceled (and instead, mids will form up outside the stadium). The whole thing amazes me . . . in a bad way.