Not alowed to buy 2pair if one is missing

I was at Brooklyn and went through the sale stuff. Found a Spiderman t-shirt for my son that loves Spiderman. When I wanted to pay for it I was told that there is no way I can buy it since its a 2pair item and the other item was not attached. I was totally willing to pay the price for only one item but was told there is no way I'm able to buy it! (Surprise woolies, half of the children's section that's on sale has half of the pair missing!!) I don't understand how it can effect you if I pay the price of a 2pair and only receive one..