I have had that experience with many companies. Often times in financial companies it will be up to the discretion of a partner to decide if they would like to pay interns.
I interviewed with two different offices of a large brokerage (shall remained unnamed but they were purchased by BoA). One office told me I would get school credit for filling orders. The other office offered to pay me for doing market analysis. It seems the difference lies in how much they value an interns service.