Because Congress voted to let them do it 2008.
Fisa court oversight: a look inside a secret and empty process | Glenn Greenwald | Comment is free | guardian.co.uk
When the original Fisa law was enacted in 1978, its primary purpose was to ensure that the US government would be barred from ever monitoring the electronic communications of Americans without first obtaining an individualized warrant from the Fisa court, which required evidence showing "probable cause" that the person to be surveilled was an agent of a foreign power or terrorist organization.
That was the law which George Bush, in late 2001, violated, when he secretly authorized eavesdropping on the international calls of Americans without any warrants from that court. Rather than act to punish Bush for those actions, the Congress, on a bipartisan basis in 2008, enacted a new, highly diluted Fisa law – the Fisa Amendments Act of 2008 (FAA) – that legalized much of the Bush warrantless NSA program.
Under the FAA, which was just renewed last December for another five years, no warrants are needed for the NSA to eavesdrop on a wide array of calls, emails and online chats involving US citizens. Individualized warrants are required only when the target of the surveillance is a US person or the call is entirely domestic. But even under the law, no individualized warrant is needed to listen in on the calls or read the emails of Americans when they communicate with a foreign national whom the NSA has targeted for surveillance.
As a result, under the FAA, the NSA frequently eavesdrops on Americans' calls and reads their emails without any individualized warrants – exactly that which NSA defenders, including Obama, are trying to make Americans believe does not take place.