Google is a privately-owned company and it has the right to do whatever it wants within the law, but hopefully it's reasons for banning certain sites are sound and consistent. If it's banning sites that promote violence, that's perfectly reasonable. If someone wants to find sites like that badly enough, either for their own personal interest or for research, I'm sure they can do it without Google.
But if Google is just banning groups with particular opinions or beliefs (political, religious, etc), whether they're violent or not, then that's different. Freedom of speech is supposed to be granted to all groups as long as they don't promote the violation of someone else's civil liberties. But, like I said, Google is a private business, so doesn't that mean they have the right to "refuse service" to anyone?
Does Google ban sites that are related to al-Qaeda or other terrorist organizations? Do they ban sites that teach people how to make bombs? Do they ban communist websites that promote world revolution? Those aren't rhetorical questions, I really don't know.
Unfortunately since it is google they can do as they please... but if they do decide to lean to far one way or another they could get in big trouble meaning people stop using google and go to MSN or Yahoo.