Pure and simple. If we start violating the constitution and throw away our civil liberties, then we are worse than the terrorists. This country is founded on certain beliefs, and these must never be violated, even if it might make it a little easier to eavesdrop on terrorists. It makes it legal to eavesdrop on anyone, and that's wrong. We can't trust the government to have this kind of power, especially when they could apply for a warrant from the FISA court. The only reason they want to do this is so they can keep their activities secret from Americans, it has nothing to do with protecting us from from terrorists.

If wiretapping US citizens in the US can help prevent terrorism, fine, do it, but why not get a warrant for it? Judicial review is one of the cornerstones of the American system of checks and balances that keeps us from sliding into tyranny. Warrants can be obtained in a matter of hours. I find it very hard to believe that simply requiring warrants to wiretap US citizens would limit our ability to fight terrorism at all.