It definitely wouldn't be either Microsoft or Bungie, because Gathering of Developers is the publisher. However, retailers decide what to set the price of a game at. It's no small secret that retailers generally charge more for a Mac version of the product, simply because they're not going to sell as many copies compared to a PC product that could be on that same shelf space.
Most users are better off ordering online where you can find better deals and better service from retailers who value their Mac sales a lot.