I'm not sure that's taunting--though it's clearly designed to look that way.
I think it has something to do with the communication between the government (probably WH) and the shooter(s).
I think the shooter(s) has been leaving demands for the release of Ahmed Abdul Rahman with the tarot card (The Sword), Ahmed's nickname. I think that's why they won't show us the card. (And it could be that showing that card would be the signal for other cells to start up, too.)
There's a negotiating process going on-all done in code--which is why Moose has so many press conferences, and why he is the one who is always giving them. He's the government's spokesperson, relaying the message in the negotiation. When the shooter is not pleased with the answer, he kills and leaves another message.
I know that's not much of an explanation, but it's the best I can do right now. I just know that they are negotiating through those conferences.