I have no Idea but I think it depends on where you buy it. And I don't know what you mean by bonus items since im not interested in RAGE.
But if you mean those extra sidequests and areas, then it doesnt matter if you pre-order or not. You probably know those online passes for Games like NHL or Fifa? (Mostly EA games). RAGE has the same thing but for the campaign.
It means that you only get the code if you bought a new copy of the game since used copies wouldn't have the code anymore.
So they just want people to buy the game because of those extra quests and areas or whatever.