It came after he was held down by Eels hooker Reed Mahoney and reacted, lashing out and connecting with two slaps.
Smith spoke on Channel 9 after the game and said it was the right call "as long as it's consistent".
"He just wouldn't let me go and I was just trying get his grip off my jersey and I couldn't so that was unfortunate," Smith said. "I spoke to him a halftime, a little exchange in the tunnel after the first half and he had a little wry smile and said 'got you there', so 1-nil to Reedy but just happy with coming back on and how the boys responded for that 10 minutes, it was a really crucial 10 minutes. I was really disappointed with myself and that I reacted that way and left the side vulnerable with 12 men."
Smith said he was "antsy" in the dressing room.
There was plenty of debate over the sin bin.
Andrew Voss said "an absolute brain explosion here in the semi final with a referee not metres away, centimetres away".
Phil Gould exploded that it was "an overreaction" and "over officiating".
Gould said he was trying to slap at his arm and caught him in the head.
"It's an absolute nothing, an absolute nothing, an absolute nothing, an absolute nothing – what a disgrace," he said.
Post-game, Gould said he was angry about the decision.
"The game's a nanny state," he said. "This governing body is so over protective and interfering with the game they've developed an era of referees that are so over coached and the game is over refereed and this is ridiculous. No one should be sin binned for this, I defy anyone to justify it, I really do. It's not what the fans want, it's not what the players want.
"Honestly. Everyone comes to me because I'm willing to say it but it's a farce, this whole thing with the refereeing over the last five or six years is a total farce. Sooner or later we'll get someone in control of the game who will have the courage to end it, to stop it, to get rid of these people so we can get our game back."
Asked if it was him, Gould said "they wouldn't let me within 100 miles of head office".