Costco aren't able to sell that in the EU or 25 other countries including the UK and China, which still respect EU food regulations. Of course, that includes Greece, although Costco don't have a presence there. It even includes China where Costco sells genuine Greek Feta.
By law in the EU and EU regulation respecting countries, due to its PDO status, Feta can only be made with sheep's milk or a blend sheep's and up to 30% of goat's milk.
Feta-like cheeses can be made, but the name can't be used. Salad cheese is one such rename I've seen a lot.