when a client goes short via a spread bet the SB compnay looks at it slightly differently. As far as the SB company is concerned you (the client) have lent us the stock. Therefore we pay you for the privalege of 'borrowing' from you.
In quarterly spread bets this is hidden in 'time decay'. Which means that if a stock was trading at 100 and you sold a six month forward spread bet in that stock AND interest rates were 6% AND the stock paid no dividends. THEN the price that the spread betting comapny would quote would be around 103 (half a years interest at 6% on 100). If the stock did not move for the entire six month period then the price would decay down to 100 and the holder of the bet would make a profit even though the share had not actually moved. Of course the holder of the bet would lose a certain amount of value from the margin that he/she had put up to cover the position in the first place.
In rolling bets the fee is more upfront and the SB company will pay either an amount each day or by a slight change in the bet price to a holder of a short position in a stock.
If a large number of clients short a stock then , yes, capital spreads will short the stock via its brokers (we are members of the LSE) and will pay borrowing costs to a stock lender, but we will earn interest on the funds received from selling the stock.
Simon