we get shafted.
Because Bethesda decided it didnt care about 2/3rds of its fans.
It's not because they don't care about the fans so much as they do care about money, and the idea was to sell more Xbox copies of the game and therefore more Xbox DLC, so there's a deal in place where Microsoft gives Bethesda a few extra bucks and everyone wins. Also, I doubt it's 2/3rds exactly. PC gamers don't generally make up for a full 3rd if we're talking Xbox/PS3/PC gaming.
Also who's fault is it? They announced before the game came out that the DLC would be exclusive, so... if you had an Xbox360, you had the choice to buy it for that platform. If you only have one or the other (understandable, Consoles are expensive, especially for PC gamers that are sinking $400 or more a year into upgrades to play new games), or are some really devoted fan that believes they can only have one system and owning the other is the same as treason, well... Bethesda didn't make that decision for you.