The U.S. government concedes that it needs a warrant to eavesdrop on phone calls between Americans, or to read the body of their emails to one another. Everyone agrees that these communications are protected by the Fourth Amendment. But the government also argues that Fourth Amendment protections don't apply when an American calls or writes to a foreigner in another country.
Let's say, for example, that the head of the NAACP writes an email to a veteran of the South African civil-rights struggle asking for advice about an anti-racism campaign; or that Hillary Clinton fields a call from a friend in Australia whose daughter was raped; or that Jeb Bush uses Skype to discuss with David Cameron whether he should seek the 2016 presidential nomination for the Republican Party. Under the Obama administration's logic, these Americans have no reasonable expectation of privacy with regard to these conversations, and it is lawful and legitimate for the NSA to eavesdrop on, record, and store everything that is said.
The arguments Team Obama uses to justify these conclusions are sweeping and worrisome, as the ACLU's Jameel Jaffer captures in his analysis of the relevant legal briefs:
... the government contends that Americans who make phone calls or send emails to people abroad have a diminished expectation of privacy because the people with whom they are communicating—non-Americans abroad, that is—are not protected by the Constitution. The government also argues that Americans' privacy rights are further diminished in this context because the NSA has a "paramount" interest in examining information that crosses international borders.
... the government even argues that Americans can't reasonably expect that their international communications will be private from the NSA when the intelligence services of so many other countries ... might be monitoring those communications, too. The government's argument is not simply that the NSA has broad authority to monitor Americans' international communications. The US government is arguing that the NSA's authority is unlimited in this respect. If the government is right, nothing in the Constitution bars the NSA from monitoring a phone call between a journalist in New York City and his source in London. For that matter, nothing bars the NSA from monitoring every call and email between Americans in the United States and their non-American friends, relatives, and colleagues overseas.
All I'd add is that the Obama administration's encroachments on the Fourth Amendment disparately affect naturalized citizens of the United States, almost all of whom still have friends or family members living in their countries of origin. When I call my parents, email my sister, or text my best friend, my private communications are theoretically protected by the Bill of Rights. In contrast, immigrants contacting loved ones often do so with the expectation that every word they say or write can be legally recorded and stored forever on a server somewhere.
Xenophobia is one factor driving this double-standard. It does real harm to immigrants whose speech is chilled, as is clear to anyone who has made an effort to speak with them.
Yet there has been little backlash against the Obama administration for affording zero constitutional protections to Americans engaged in speech with foreigners, and little sympathy for the innocent Americans, many of them immigrants, who are hurt by the approach Obama and many in Congress endorse.