While it's not a 'free' download for people who have already purchased the application, I think that Microsoft are playing fair. While the game may feel similar, Spartan Assault will have required some coding effort to port over to the console, and that doesn't come free.
And if you're an avid Xbox gamer, note that the Xbox One version of the game will have 1000 Achievement Points to pick up, as well as the addition of a new co-operative game-play mode.
I'll be interested to see how the different viewing angle translates to a home console, given my thoughts in the initial review here on AAWP:
Spartan Assault goes for an older style of action gaming - the top down view. Strictly speaking, it's about 65 degrees down, so there is a touch of perspective on the landscape, giving the characters and terrain a convincing three dimensional look. While the characters are relatively small, this does give you significantly more vision over the terrain, which makes for a better game. I also like that the view zooms out a little more when you jump into a vehicle or fixed gunnery position so you can see even more.
Halo: Spartan Assault is still available in the Windows Phone Store, and the Xbox One version is just two short weeks away....