Then buy it. But the promo does have pieces of music that the score CD will not, so I can understand people downloading it. If they download it and don't buy the score, they likely weren't going to anyway.
People think that this is going to be a huge seller, but we have all seen that internet fads (Snakes on a Plane) don't always translate well to actual sales. A lot of people probably wanted an official score release just so they could eventually download it from torrents or what not.
If people really were going to buy this release, they will probably still do it regardless. But thinking the score is going to sell well enough to warrant a 2 CD release is very wishful thinking. I'm still waiting for a full release of Star Wars Episodes II and III so the wait will likely be quite a long one for Transformers as well (if it ever happens).