completely fair. Playstation owners never had a Halo game to play ever, it was a Microsoft exclusive franchise ever since Combat Evolved (maybe PC got some, i can't remember).
anyway, this is a new franchise with a new partnership, and its not like Xbox owners have to buy a PS3 or PS4 to play Destiny, they just have to wait it out a bit for some timed exclusives to hit the marketplace just like every game with bonus or pre-order content these days. as for the beta, i guess it's just a part of the contract they signed to be able to get all that cash they've needed to attempt to make such an ambitious game.
i'm a 360 owner, and i'm happy that i've been allowed to play this thing even for a few days before it's comes out mere weeks later. we should all be happy we're actually getting to play it even for a short time and helping to shape the final release in a few more weeks :)