Rockstar Games is part of a publicly traded company. Its only duty is to its shareholders. While it usually can't hurt to please the public, they will do whatever they think will make them the most money. That's it. There are no other goals or objectives.
Microsoft gave Rockstar a $50m advance on revenues from the two episodes, on the condition that they were Xbox 360 exclusive. At the time, Rockstar wasn't exactly in a very good position - it had just had the Hot Coffee scandal, its share price was at rock bottom and a huge number of its staff had walked out or left, many seeing San Andreas as the high point of their careers. It's a huge sum of money to get up front and I would probably go as far as to say that without this advance we wouldn't have ANY content at all.
So you can take your pick... DLC only on Xbox 360 or no DLC at all. If you're really bothered by it, buy an Xbox 360. You won't regret it because there is a lot more than GTA DLC exclusive to that console.