See the Beta being exclusive as a gift to loyal members of the community who are envolved enough to spend money on the game before it comes out just to be playing since day one. Betas aren't a demo for every player out there. This isn't about marketing, it's about testing the actual game and getting it ready for the launch date. Also, yo can't forget that the Beta isn't just free for Bungie. It implies a lot of investment on money, time, and many employees being focused on it. They need to secure they are at least covering the costs of the whole Beta, plus it's a great way to lure players to buy the game and get them involved as soon as possible in the community and development of the game.