Cameras are used by all kinds of private homes and businesses as security, many security cameras are actually installed to record acts of theft or violence against private or corporate property.
And cameras in government buildings aren't any different, as they tend to be more targeted for attack.
Do you suggest we just let drunk drivers and people who hit-and-run pedestrians go free? Does that validate your sense of personal privacy?
I'm not saying that invasion of privacy is a good thing, but the usage of technology to protect ourselves is not merely a government phenomenon, it's widely privatized, and also widely used for other things other than acts of terrorism.
Also the invasion of privacy mentality has more to do with things like reality tv, youtube, and the Internet than the government.
Settle down out of your ideological bias and try to view this more clearly.