I don't actually really know the full answer to that. If I had to guess, I would say that there generally need to be pretty distinct objective lines for when something becomes a crime (you can't have like a weight limit or something to where "throwing a sandwich" isn't a crime but then transitions to a crime as the object gets more and more hazardous up to where throwing a rock is a crime.) I think it is generally that it winds up being a sensible system just because simple battery just really isn't all that serious a charge. You can get convicted, in theory, just because you swatted someone's arm away, or threw a cup of water at them, or whatever, and it really just won't impact you all that much. (But then, if it's a rock or if you injure the person, or something like that, then it's a different and more serious charge than simple battery.)

Again, the trouble comes in when you're dealing with the cops and do one of those "simple battery" things and all of a sudden it comes with life-changing consequences.