Devil's advocate...
The main sticking point probably has to do with the distributor's (MS, Valve) cut. Giving XBOX licenses to backers likely costs something to Frontier.
Frontier gave away steam keys to existing customers which I imagine had some retroactive cost collected by Valve (I could be wrong, just a strong hunch).
Now, steam is just another distribution platform just like Xbox (I know, a stretch, I'm being a contrarian on purpose) so why shouldn't they extend the same option to backers wanting to play on Xbox as they did with Steam?
Disclaimer: I think doing that would cost FD way too much money but I am trying to outline the logic leading to this being perceived as unfair as it is currently. However, I would love to live in a world where game makers held all the keys. I want to live in a world where I can drop 70 bucks on some intellectual property and then be able to play it on any platform or multiple platforms that I choose.