why do i have to pay for premium when i bought the limited edition on the day battlefield 3 came out????????????????

OP the limited edition  explicitly gave you free access to the Back To Karkand expansion and nothing more.  The Premium option gives you access to all of the expansions at a reduced price, early, with extra perks such as double XP, camos and assignments that non-premiums do not get.  Also you didn't have to pay extra for the Limited edition at launch, all first run copies of BF3 were Limited Editions that sold for the same exact price as the later non-limited edition that did not have free access to the Back To Karkand expansion.