I remember it was stated policy that they did not pay interest. i remember reading that with utter disbelief. That said to be fair they were charging a much lower cost of borrow than anywhere else retail and likely the policy was aimed at rewarding those who generated comm for IB with lower borrow cost subsidised by not paying on inactive cash balances. They do now pay interest likely because when interest was 25BP or whatever the fact that somebody else might be paying 25BP was likely not enough to make someone move an account. As interest rates increase it becomes more of an issue.