I'm pretty sure that the government gets a court order and actually goes a step further by not simply blocking a link, but having the hosting provider shut the site down completely (Vandy Vol, or other lawyer types, might be able to answer this definitively).
The government can and does shut down websites in the United States which host child pornography. Google does not legally have to block links to child porn. This is because, as a search engine, they sort of get an exemption; they're just reporting what's out there on the internet by maintaining a cache database of sites.
If Google were just a random website, and they specifically placed a link on their site to a child pornography site, then they could likely be charged with distributing child porn, and thus their site would have to be taken down (or at least have the link removed). But because Google or any other search engine simply links to websites based upon user input in the form of a search, it's not like they're actively promoting a child porn site with a static link.
Essentially, my understanding is that Google (rightfully) gets an exemption due to the nature of its website, but that any other website located in the U.S. hosting and/or linking to child porn will have its owners prosecuted criminally and the site itself will be shut down.