Saddlers' midfielder Sam Mantom kicked the ball out of play so that an injured Blades player could receive attention after being hit in the face by the ball.
But instead of knocking the ball back to home goalkeeper Sam Johnstone when play resumed, Porter hammer the ball out of play near the corner flag and United went close to scoring.
Smith said: "That was disappointing. Their centre-forward decided not to give the ball back to us, which upset my players and that changed the momentum.
"The lad apologised to me as he came off at the end, but it would have been interesting if the shot they had soon after hadn't gone over the bar.
"In the dressing room there is a feeling amongst my players that they have got beat because of all the chances we had in the first half. We should have been out of sight. It was a really good performance and I'm disappointed we didn't get a win.
"But I have told the players theirs is a fantastic achievement and we still have something to play for with three games left. On another day Febian Brandy would have scored two or three goals."
Craig Westcarr gave Walsall a 44th-minute lead with a low shot from James Baxendale's cross.
But Brandy missed a hat-trick of decent chances before the Blades improved in the second half and grabbed a point when Kevin McDonald's shot was turned onto a post by Johnstone, and Porter tapped in the rebound.