Anyone ever try to enter a Costco store barefoot? The person at the entrance checking your membership card won't let you in, so don't even think about it. The food court area outside the main store, however, doesn't have a problem with it. At least, this applies to the Costco in my city, and I assume all the other Costco stores are the same.
... so if they don't want your money, just spend it somewhere else. Their own problem ;-) A company who doesn't esteem their customers, isn't worth to frequent. Easy.