I agree. It isn't a scam. IB is sacrificing some of its interest income, because it wants to promote SSF trading and to enjoy the resulting income stream from SSF commissions and from IB's investment in the SSF exchange, Chicago One. This provides customers a good deal. SSFs enable customers to earn higher interest on cash balances and short sale cash balances than any broker will pay, and to finance long stock positions at lower margin interest rates than any broker will charge.