You overestimate the drawing potential. CO(villian) has already 3bet this flop, and it is heads up by the time it gets back to hero. Co's 3bet reduces the chance he has a draw to charge, and increases the chance CO is trying to prevent SB (and hero) from drawing. CO might have A4, A[img]/images/graemlins/spade.gif[/img]3[img]/images/graemlins/spade.gif[/img] or 45 and/or two spades, but I think he is likely enough to have nines, that hero can sacrifice a small bet on the flop to foster action on the turn.