The most direct reason is that a federal law that those who host, rather than author, speech on the Internet cannot be treated, for legal purposes, as having published it….
It’s important to realize that even without this law, Google probably would win any defamation suit brought against it anyway. A defamation claim requires not only proof of publication, but also proof of state of mind. And Google would virtually never have the state of mind required — or, indeed, any particular state of mind at all, with respect to a particular item that comes up in a given search. Since its searches work automatically, no one at the site would likely be aware that Google’s search engine had displayed a particular false statement of fact.
The liability of bloggers and and website operators like Drudge for re-published defamatory statements remains unclear. And when defamatory statements originate on a blog, as CNN notes, “many webmasters, bloggers, and the like are either anonymous and/or judgment-proof — that is, too poor to make a lawsuit worthwhile.” (Via Moorish Girl.)
But a former law school professor of mine has written extensively about the many lawsuits brought by large corporations to silence individuals criticizing them online. These lawsuits are called SLAPP suits.