The ones I get from strings & beyond each string comes in a sealed plastic package there should be no corrosion, However because of the holidays there has be a lul in gig's and even rehearsals so the strings indeed have been sitting around a while, I think I ordered them back in July... so yeah corrosion makes a lot of sense.
I use d'addarios exclusively and since a recent move most of my guitar's strings flash rusted because it rained and there was an open window... That in turn made my fingers black from the corrosion and I guess the nickel plating wearing off as well...
My case was user error and aside from that Ive never encountered a problem with them. I also agree that string counter fitting wouldn't be very lucrative and I don't see it being an epidemic or a current concern... As always it's best to research your supplier and the product prior to purchasing.. Its also best to keep your hands clean and your strings dry.