It's limited content for a reason, though, and also regional for a reason.
In the EU we don't get a lot of these offers for various reasons, but generally it's because the cost : reward ratio is a lot lower than in the US. EA might not want to do the promotion over here because the advertising would be a lot less effective, and Dr Pepper might not want to do it here because their product doesn't sell as well therefore they'd not see a decent return on the deal.
By definition, "limited" means that you can only get it if you fulfill certain criterea - Location, date, purchase of a product etc. If they made all DLC available to everyone, it destroys the idea of "limited" and it would anger more people than creating the items did in the first place. Most of us here would have paid full price for ME2 and gotten a small bit of DLC for getting it pretty much on the day (Inferno or Teminus/Rifle) or for taking advantage of an offer (Incisor for DDE, Dr Pepper items, Collector Armor/Rifle), and if EA/BioWare turned around in a few weeks/months and said "Right, everyone gets all the bonus DLC" then a lot of people would be annoyed, and I mean a lot. We got that content when it was limited and if it was made available for all, it loses the status of being "limited". I paid £40 for the DDE (most preorders for the standard edition were £27ish) and because I bought the better edition, I got a set of armor and 2 rifles (collector + incisor) along with the other bonuses.
Why should someone who buys the retail edition four months later get those bonuses? By all logic, they shouldn't. People who bought the Collector's Edition got a nice case, an art book, a mini comic... Those who bought the DDE have two PDFs and a listing on Steam/somewhere else that says "Mass Effect 2", with nothing physical to show for it. The addition of the Incisor Rifle, IMHO, was to help make up the difference between the CE and DDE. As the 360 doesn't get a DDE, it should be exempt from the rifle.