I think some of the thinking behind it is...
bonding is important for our group,
people get to know each other and bond faster in adverse situations
ergo, let's all create some unnecessary adversity!
I do understand the need to share experiences. Getting caught in a rainstorm is an example of something that is difficult but an interesting experience to share and something to talk about later. Having a stranger throw something at you whether you want it or not, is not a shared experience, it's a form of assault. If you were throwing a water balloon at a politician or celebrity to make a political point you would probably be arrested, so this is also an example of "those crazy college kids" being given special treatment.
Water coming at you is very much a sensory issue, which is why I had such a love/hate relationship with swimming pools when I was a kid. I loved floating, and the sensation of weightlessness, and even being surrounded by the water, once I transitioned into it, but there was always some kid who would get off on splashing everyone else, which was always overwhelming. I think NTs need to know that for someone with sensory issues, such things are not so much "annoying" as overwhelming and its about as obnoxious as tripping someone with crutches.
@Determinism concerning the 75% extroversion, and @Tenzinsmom, concerning "What you call NT, I call Western":
That 75% is, as far as I can tell, among NTs in the United States, so you will find different numbers in other cultures. In Japan, for example, there are more introverts than extroverts. Of course, I would also guess that there are differences between cultures in the percent of people on the spectrum and in definitions of what would be considered NT. Tenzinsmom, if you have lived in the east, I would be interested in reading about some of your experiences there.