No matter if you agree or not, Sega did it and they have solid reasons for that. According to game’s lead designer Artem Kulakov: “DX10 has offered a lot of advantages over DX9. First of all, DirectX 10 allowed us to simplify the rendering engine. It matches capabilities of next generation consoles better than DX9, which is important for us considering that Stormrise is a multi-platform title. We had fewer driver-specific compatibility issues with Stormrise compare to our previous games released with DX9.”
Maybe the company just wanted to make sure that everybody who has XP sticks to Empire: Total War and didn’t find any other way of putting it. Either way, I’m sure this move will cost the developers quite a few users… Personally I don’t like how gaming and Vista work together.