The key here is a legal principle known as the "third party doctrine," which says that users don't have Fourth Amendment rights protecting information they voluntarily turn over to someone else. Courts have said that when you dial a phone number, you are voluntarily providing information to your phone company, which is then free to share it with the government.
This all dates back to a 1979 Supreme Court decision. Police had asked the phone company for information about the numbers dialed from a robbery suspect's phone. The suspect objected, pointing to a famous 1967 ruling holding that the Fourth Amendment requires a warrant to record the audio of a phone call. He argued that the same principle ought to apply when the government records information about the numbers a suspect dials.
The Supreme Court rejected this argument. "We doubt that people in general entertain any actual expectation of privacy in the numbers they dial," Justice Harry Blackmun wrote for the court. He pointed out that telephone customers are used to seeing numbers they've dialed on their monthly telephone bill.
Blackmun's reasoning may have turned on the fact that automatic dialing was a relatively new development in 1979. Previously, telephone users had to tell a human operator which number they wished to reach, making it plausible to regard the phone company as an active participant in the phone-dialing process, but a mere passive conduit in transmitting the phone call itself.
Technological progress has rendered this distinction increasingly dubious. For example, cell phone companies now keep records about the locations of their customers' phones. The government has argued that this "non-content" information should be available without a warrant. Yet such records amount to a detailed record of everywhere the phone's owner has been in the past month; a much more intrusive form of surveillance than a list of the phone numbers a customer has dialed.
The third party doctrine also suggests that the users of cloud e-mail providers don't even enjoy Fourth Amendment protections in the contents of their messages, since those have been voluntarily shared with third parties such as Google and Microsoft. One appeals court has ruled that a warrant is required, but so far most other courts have not followed that precedent.
Journalists get a bit of a special deal here. The government has established special policies to guard against inappropriate surveillance of reporters. Before an FBI agent can seek a journalist's call records, they must get special approval from the attorney general. But that's merely a Justice Department policy, not a constitutional requirement. The policy could be changed in the future, and the lack of independent oversight makes abuses more likely.
The rest of the country doesn't get even the modest procedural protection the government affords to journalists. The FBI likely didn't need a warrant to obtain e-mail records that led to the identification of Paula Broadwell as the mistress of Gen. David Petraeus last year. Police officers have become increasingly aggressive in seeking records about the locations of suspects' cell phones.
Merely adding more safeguards for journalists' call records won't fix the underlying problem. As more and more information about us is held by third parties, the court's cramped interpretation of the Fourth Amendment leaves ordinary Americans with less and less privacy. People may or may not have expected the numbers they dialed to be private in 1979. But they certainly consider the contents of their Gmail accounts and the locations of the cell phones to be private information today.