If he were trying to plow over a defenseless player, go at the knees of another player or simply having his spikes up in the air, I'd have a real problem with the play. But he did none of those things.
Simply sliding in at a player can give a split second for the slide to be completed as the natural instinct is to brace for contact. Heisey probably thought he was dead to rights so that's likely what he was doing. In hindsight, if he slides away from contact, he would have certainly been better off in that situation.