Nieman Lab’s Zachary M. Seward explains.

Also: at the White House press briefing just now, MSNBC’s Chuck Todd asked Robert Gibbs whether the White House would apologize to New Yorkers for its part in sparking this “terrifying tweet.” Gibbs did not have an answer at the ready.

Another reporter just asked whether the White House had been “insensitive” to New Yorkers. Gibbs will get back to reporters on that.