And it would be a simple matter for the government to get a warrant to monitor any communications you make over your telephone"
This quote from Byron Yorks column presents the argument for the policy and knocks down the 'simple matter to get a warrant' response:
Lawmakers of both parties recognized the problem in the months after the September 11 terrorist attacks. They pointed to the case of Coleen Rowley, the FBI agent who ran up against a number roadblocks in her effort to secure a FISA warrant in the case of Zacarias Moussaoui, the al Qaeda operative who had taken flight training in preparation for the hijackings.
Investigators wanted to study the contents of Moussaoui's laptop computer, but the FBI bureaucracy involved in applying for a FISA warrant was stifling, and there were real questions about whether investigators could meet the FISA court's probable-cause standard for granting a warrant. FBI agents became so frustrated that they considered flying Moussaoui to France, where his computer could be examined. But then the attacks came, and it was too late.
Rowley wrote up her concerns in a famous 13-page memo to FBI Director Robert Mueller, and then elaborated on them in testimony to Congress. "Rowley depicted the legal mechanism for security warrants under the Foreign Intelligence Surveillance Act, or FISA, as burdensome and restrictive, a virtual roadblock to effective law enforcement," Legal Times reported in September 2002.