BT also crits twice as often MS. When you put that together with a 25% shorter CD, BT doesn't come out too far behind MS.
Edit: Actually because of the high CS uptime of Arms MS does come out ahead by a pretty large margin. But Fury does do more damage overall so it doesn't really matter. Arms doesn't scale as well with gear so it's not so bad that they get a little better use of the 2-piece.