I don't practice criminal law (currently) but have in the past.

I would note that it doesn't appear that this was thrown from the "cheap seats." He likely comes from a family with means, which means that he will be held less responsible than some less affluent kid. I hate that about our system, people are often excused from their crimes because of the monetary resources that can be brought to bear. The Book of Mormon has things to say about this, and it's not complimentary.

From what I've seen, he intentionally threw the water bottle, and the video shows it coming in like a missile as has been stated elsewhere on the thread. I don't think anyone would have a hard time getting a jury to come to that conclusion unless there is someone on the jury who is more worried about giving an 18 year old a record rather than actually enforcing the laws of the state.

Which leads me to observe that "good kids" don't throw water bottles full of water (whether they are plastic or not, clearly there was some mass to that thing) towards other people unless they are acting in self defense. This is NOT a good kid based on his actions, and those actions should have consequences. The law passed by our democratically selected representatives condemns this behavior and proscribes a specific punishment for it.

If you want to have the law to not punish someone who engages in such behavior so that they can go on a mission or join the military, then lobby your state rep or senator. Get them to add a "good kid/get out of jail free" card to the law. Or, you can understand that it is our laws, and their enforcement, that allows us to have a civil society. If a kid with rich parents gets a free pass on this, what does that say to the kid with poor parents? If a 18 year old gets a free pass in this situation, then why should any freshman at the U feel any deterrence from engaging in the exact same behavior? But, throw this kid in jail and it gives everyone else a reason to not throw dnagerous objects on the field.