Be serious - you can't possibly expect any game company (or non-game company, for that matter) to allow someone to use your intellectual property to create something the directly competes with you. And before you say that it's PC or it doesn't compete because it's free, things don't have to make money themselves to impact the sales of a similar product.
We're talking about HALO here. Not all RTS games...not even all sci-fi RTS games. There is simply no way anyone has the right to make a Halo based game without Bungie's (Microsoft's) permission. And there's no way a responsible company would allow it, especially when it could impact their own sales.