So what is it exactly that makes so many liberals enjoy physically attacking conservative speakers?

Not too long ago, it was Ricahrd Perle - Protester Throws Shoe at Richard Perle (video here)
Then, William Kristol - Conservative gets pie in eye at Quaker college
And most recently, Pat Buchanan - Pat Buchanan Attacked (video)

Oh, I know there are those who think this type of thing is pretty funny. But is that the appropriate response?

Now I don´t like Pat Buchanan very much, but when these are invited guests in what is supposed to be a forum for the exchange of ideas, shouldn´t they feel safe from physical harm?

Now I´m not going to be a Krugman and predict the political affiliation of the next person to commit an assassination, but he seems to ignore the anger of the wackos on the left.

If anyone can let me know of a recent appearance by a liberal pundit or politician which led to a personal attack, I will be glad to update this post with the appropriate denunciation.