This is really a PR issue. Announcing it before the game is released usually leaves a bad taste in people's mouths because they haven't even gotten to touch the main game yet. Paid expansions are fine but they need to release the details after the game is out.
To the people wondering why they're announcing this before release. I imagine it has a great deal to do with being able to remind people of the expansions when they go to buy the game. This way they need to do a lot less marketing for the expansions.
People that like the game when they play it will essentially be reminded that there will be expansions available later down the road.