Slowplaying a set on the flop would be a +EV decision, since Hero isn't representing a drawing hand MOST of the time when he leads out with a bet that would be a marginal call with a drawing hand. So, calling that bet (when you can assume villan isn't on a drawing hand) is +EV since you will probably face another bet from a weaker hand on the turn than by raising and defining your hand when you are already so far ahead and risk losing your opponent. He raises the turn with a set because if he calls he probably isn't extracting too much more money from hero other than with a small value bet. This is the only chance villan has to convince hero he might be bluffing. This is a set line, and very likely is not a drawing hand.