The reason why you would find fakes at a Marshalls is because Marshalls like some other big chain stores have a no hassle return policy. You can basically return anything to them even if you dont have proof of purchase or if you never bought it from them. Given the new conditin the store doesnt have a shoe specialist that tells if it is fake or not they just retag and put it back on the floor. There are stories of people returning things like tires to Nieman Marcus.
Best Buy and Wal Mart used to have similar policies but I do not know if that has changed over the years.