I think they should just announce the changes and push them out at the same time with no prior warning. Why do we need a warning? What does it change knowing that a thing will happen in a couple of weeks instead of now?
There's nothing I hate more than playing against OP cards which Blizzard themselves have decided are too good for their mana cost during the two weeks (or more) between the announcement and the nerf. It just feels like the opponent is cheating!
As for why they do it they way they do, I'm sure they have their reasons. Some may be cynical and some may be practical, but at the end of the day only Blizzard knows.
It’s always about money. They’re purposely vague on which cards will be nerfed so people will still spend money on packs and battle ready decks, then they pull the rug out from under us.