Why should we be surprised that the theater attack in Moscow was copied? Well, we should not have been surprised if we would have been warned as the FBI and DHS had noted in this internal document.
Terrorists’ Interest in Attacking Theaters and Similar
DHS and the FBI knew about a theater style attack (Official Government PDF Document)
On 7 April 2012, an al-Qa‘ida-linked violent extremist urged others to
emulate the 2002 Moscow theater hostage attack—in which Chechen terrorists
seized the Dubrovka Theater and held over 800 people captive for three days
until Russian security forces gassed the theater—by seizing crowded facilities in
the United States, including movie theaters; taking hostages; and demanding the
release of captured al-Qa‘ida-linked or -inspired violent extremists.