Some stores are confused, because when we first went to the gift card system (vs, the old paper gift certificates) Disney$ WERE excluded, as were passes. Now, the policy has been adapted to allow the purchase of both these items with Disney gift cards. The merchandise credits, that guests receive for returns with no receipt, and for other situations (not having same credit card used for purchase, etc) can only be used for merchandise.
It also is complicated by the fact that we don't often HAVE guests wanting to buy Disney$ with gift cards........... it is pretty rare. And with the cast turnover what it is, there can be some confusion.
It also is possible the gift cards you are speaking of, UPromise, have an exclusion on them for Disney$.