In this case, I think that calling Arrowhead a standalone game is misleading. No one buys Arma 2 to play alone - but that's not my point.
Tons of people will buy ArmA 2: Arrowhead thinking they can play online with it, only to discover that all the servers will be running a mixture of Arma 2 and Arrowhead, thus alienating Arrowhead users.
In reality, there will be no online play for those who buy Arma 2: Arrowhead as a standalone game. Why promote it as such? So people will buy it and then be forced to buy Arma 2 if they want to play online. Why not tell people the truth. If you want to join online servers, you'll need both games.