2 things, First, the Cop was just walking in to pick up security video of The previous robbery so it's doubtful he was geared up as he would be "On Patrol" ( No club or Tazer ).
Second, until he got the thief turned around he had no view of whether he was armed or not, so drawing his pistol was a better idea than not drawing it.
2 other things, Had the cop been more aware he would have seen what was happening through the window, or at least from the doorway, second, He never should have approached a potentially armed bad guy the way he did.