I honestly can't tell if leftists are being serious or not. Are they simply trying to score some cheap political points against their enemies by associating the rhetoric of right-wing activists and politicians with the actions of a looney tune gunman (and he was a looney tune, not an activist with a cogent agenda, just look at his youtube videos), or do they seriously think that passionate rhetoric actually leads people to take this kind of action?
If the former is true, shame on them. If the latter is true, then I have to question their grip on reality.
Yes, to a certain extent people are shaped by their environment and the culture(s) they live in. But the best your environment can do is bias your towards certain options, it can't actually make you make certain choices.
More to the point, if the type of words this article is complaining about caused shootings, there would be a lot more shootings. I have a far better explanation for what happened: a crazy man got his hands on a gun and had an freakout that left six people dead. This isn't South America (or Revolutionary France), where assassinations are a regular part of the political process (that was tongue-in-cheek, so hold your angry emails). Assassinations in this country are virtually always motivated by crazy people. That's probably because it is a lot easier to get rid of a particular politician in America--through the ballot box--versus a country where they have regular rotation of dictators.
Anyway, if the left is serious about what they are saying here, then they aren't going far enough. Where were they when Republican offices were having bricks hurled through windows? When left-wing activists were calling Bush a terrorist, the next Hitler, a maniacal dictator (and also simultaneously an idiot), or calling Rove or Cheney Darth Vader incarnate?
Of course, I don't expect serious thinking or self-restraint from politicians or activists. They are partisan after all, if not to a party then to a cause. I'm sure that if some nut had taken a shot at Bush then Republicans would be saying that the hyperbolic anti-Bush rhetoric played a part in it.
I'll leave you with this link. It's from the occasionally weird World Net Daily.