I am curious to see how American Internet is different from other countries in this regard. I have long had the suspicion that our love for hatred online and South Park episodes comes as a psychological defense against insane levels of in-your-face politeness, fake plastic smiles and excessive "political correctness" dominating our offline lives. Even our everyday language is affected by this: people don't die or get fired, they pass away and being let go. George Carlin said it best: soon we'll be calling a rape victim "an unwilling sperm recipient".
Perhaps the cure for online rudeness is being less hypocritical offline.
I find many people are against "fake politeness" just to have a reason to act like jerk when they know they can get away with it. Contrary to your opinion, there are plenty of people out there who are genuinely nice.
Plus, there is a difference between disagreeing with someone and spitting in their face or threatening physical violence. In the context of this article, I don't think the idea is that we need to avoiding offending people at all costs. It's that we tend to reward the "hate" to the exclusion of other forms of opinion and that we are quick to judge people at a superficial level based solely on what someone else heard from some blog they stumbled upon.
So basically, this isn't about political correctness at all, and looking through the article, I'm not sure how you thought it was in the first place.
Not that this matters, but my experience agrees with your hypothesis - I try to go out of my way to avoid bullshitting people to spare their feelings in person, and I usually feel no need to attack them online.
PC and passive aggressiveness build on each other. PC builds pent up aggression, which people take out on others who are open and not PC. Now you have another emotionally repressed PC person looking for a target to release on.
Interesting question, but it's complicated by the fact that an even more polite society, Japan, is said to have an even more polite internet. More anonymity, more group bonding, more political deference, when your theory would predict the exact opposite.
I'm not sure that this is entirely the case. 2chan has been quite disruptive to the culture. Its enforced anonymity and subsequent freedom from personal societal retaliation is a very new phenomenon there.
Except for the anonymous super free-for-all sites, like that one where you can write over images and video. I recall reading an article about a lady celebrity there who was trying to identify people smearing her online, to sue them. (Hadn't succeeded at time of writing.)
Perhaps the cure for online rudeness is being less hypocritical offline.