Baker did not "ask" Truly if "Oswald worked there."
Dear Royell,
Regardless, given the fact that
1) Baker believed the shots had come from six floors higher (i.e., the roof), and
2) Oswald was on the second floor when he encountered him, and
3) Oswald was calm, and
4) Truly vouched for Oswald's employment there,
It was logical for
Police Officer Cop Officer Baker to not waste time patting Oswald down to see if he was concealing a rifle in his pocket, but to let him enjoy is Coke-Cola and for Truly and himself to continue their potentially dangerous journey upwards.
-- Tom