They try to silence it. For example, the Federal Government has developed a habit of wanting to "move on" from debates about contentious issues like euthanaesia, carbon trading schemes, economic stimulus spending and internet filtering. Allow me to furnish you with an example: Labor's way of silencing debt dissent - The Australian
But they're not content to leave it at dismissive language. My fear is that some future Government may try to use the proposed mandatory ISP-level filter to quash debate about uncomfortable topics. And why not? They're not going to be accountable for what goes on the list anyway, and there's almost no recourse for webmasters to get their site off the list once it goes on. All Australians who disagree with the Government being in possession of a (completely ineffective!) means of deciding what they can and can not see online should speak up now before this debate is silenced as well. Send a letter to your local member (snail mail, not just email) telling them you disagree with the Government's plan.
I'll finish with a couple of poignant quotes, the first of which I found posted on Whirlpool.
"If all mankind minus one, were of one opinion, and only one person were of the contrary opinion, mankind would be no more justified in silencing that one person, than he, if he had the power, would be justified in silencing mankind. Were an opinion a personal possession of no value except to the owner; if to be obstructed in the enjoyment of it were simply a private injury, it would make some difference whether the injury was inflicted only on a few persons or on many. But the peculiar evil of silencing the expression of an opinion is, that it is robbing the human race; posterity as well as the existing generation; those who dissent from the opinion, still more than those who hold it. If the opinion is right, they are deprived of the opportunity of exchanging error for truth: if wrong, they lose, what is almost as great a benefit, the clearer perception and livelier impression of truth, produced by its collision with error." — John Stuart Mill
"Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety." - Benjamin Franklin