Like I always say, the general public wont know until its too late anyway. I know its just movie, but see Deep Impact. The government knew about that particular asteroid for a year before telling the general population. They don't want mass hysteria.
Pretty much. The main reason for holding back is because the probability of it hitting earth isn't very accurate considering how difficult it is to predict this asteroid's path from such as gigantic distance. Thus, I'd imagine the error in this study is pretty high still to cause a major panic. We can't predict accurately the exact path of this asteroid, only a ton of possible paths and calculating a probability out of it. Besides, the majority of people don't really understand this level of math anyways, so why confuse the public even more?
. Of course, as the "possible impact date" draws near, we can get a better probability then.