many people forget that Bitstamp "reserve" it's the client's Bitcoin.
Pretty much this. People forget customer's money is NOT a Bitstamp
asset. They have to cover up the losses from their income, which is based on trading fees.
I may add...and not only on trading fees. You cannot make millions from 0.03%. This is like free so they have other source of making money from this biz; maybe the price difference...