Unpaid internships and the people who support (edit - for support, read offer) them are particularly unpleasant for a number of reasons, the main one being that they are for the most part little more than a test to see if you are from a rich enough background to consider regularly talking to.
Presumably she runs a non-profit, which, for better or worse, are frequently graded on value delivered vs. administrative expenses. Paying an intern is an administrative expense.
You could say that "call[ing] out" Sandberg is unjust to her because, as you say, big banks do it too. However, (a) the injustice, in concrete material terms, is negligible in comparison to the lives of those working without pay, and (b) this logic, it seems to me, would suggest that all prosecution of individuals is fundamentally unjust unless one is able to round up each and every similar violators.
She is absolutely free to pay "market wage." We are free to mock the hell out of her.