Because
they can.
WASHINGTON—National Security Agency officers on several occasions have channeled their agency’s enormous eavesdropping power to spy on love interests, U.S. officials said.
The practice isn’t frequent—one official estimated a handful of cases in the last decade—but it’s common enough to garner its own spycraft label: LOVEINT.
The “LOVEINT” examples constitute most episodes of willful misconduct by NSA employees, officials said.
Really, it hardly ever happens that NSA agents spy on the people they have a personal interest in/grudge against. They're outliers, really nothing to be concerned about here. Except: "Most of the incidents, officials said, were self-reported. Such admissions can arise, for example, when an employee takes a polygraph tests as part of a renewal of a security clearance."
That's comforting. The NSA doesn't know what Edward Snowden might have had access to and how many documents he got. The NSA didn't even know that Snowden was accessing these documents. And now we know the agency doesn't know what information its own agents are abusing—illegally—unless those agents tell on themselves.
But there's nothing at all to worry about. There's no domestic spying program. Except if I were a politician with something to hide, something that might be hinted at in my phone calls or emails, I might be just a little bit concerned with what a rogue agent or two might be finding out about me, with no one the wiser.