It's not rude to question assumptions or to ask where someone gets their data from.
The voting system is good on HN, but it's limited. If you post something controversial, but possibly accurate, then if only people who don't like what you say vote you down, then it just shows that people don't like what you say. If you write something popular but kind of dodgy, then you'll get voted up significantly - and given nobody but yourself can see your score then your comment just shows in normal font weight. Those who vote it down can get it to a score of 1, and nobody will be any the wiser.
So the vote mechanism is very good here, and very effective, but it doesn't necessarily improve discourse. Nor does it force people to justify their views. Nor does it help progress understanding.
I've often been asked to provide a source for my views. I'm always happy to do so. Heck, there have even been a few times it has forced me to concede that my view wasn't well thought out. If anything, bring politely questioned helped me more than it did the questioner. Other times there has been the pleasant and surprising occassion where the questioner thanked me for my sources and agreed that I was probably correct.
That's hardly "rude garbage".