OK, so you're a Leftist, and you have a problem: In the marketplace of ideas, your ideas are losing. Even worse, your political opponents are coming out with TV ads that are about to shatter many of your carefully crafted propaganda items to dust. So what do you do?
Well, if you're a Leftist, then you do what comes naturally to the Left. You prevent your opponents from speaking. Lie, smear, and attack.
The real surprise here is not that the Left wants to prevent free speech, the surprise is that anyone is surprised by their actions.
Nobody prevents free speech like a Leftist. Stalin, Chairman Mao, Castro, and Chavez are the perfect examples of the Leftist ideology allowed to run amok.