Because when I hit the limit I want to finish my sentence and then edit down to 140 characters, not be forced to edit down straight away and forget how I was going to finish my point.
I like twitter's solution of letting you type past 140 but giving you a visual indicator that you're over the limit (and a letter-count). Facebook gives you no feedback at all until you hit submit and get the "too long, jerk!" error message. Then you edit down a bit, try again, it's still 3 letters too long, etc.
I could see them not having a letter-count display shown by default (since most statuses are comfortably <420 chars), but it'd be nice if a "20 chars too long" thing appeared below the text box once you started typing too much.
