I know some bank tellers who say they are taught to comply with ANY robbery instructions. Even if it's as simple as someone slipping them a note saying, "This is a robbery" with no explicit threats. If they feel comfortable, they can slip him the marked bills or dye packs, but they won't be punished if they don't. Get the robber out, lock the door, call the cops.
I could totally see the tellers recognizing the avocado for what it was and figuring it was on the cops to catch him, not theirs.