Irish referee Alain Rolland banished the flanker for a dangerous tackle on wing Vincent Clerc in the 18th minute.
Warburton said: "I'm obviously gutted, but there was no malicious intent.
"I felt like as soon as it hit him his bodyweight took control of what happened."
Warburton, 23, led Wales to the last four for the first time since the inaugural 1987 tournament.
The sending-off means Warburton could miss the next game - the third-fourth play-off in Auckland on Friday.
The 23-year-old flanker added: "I thought it was a normal tackle, next thing I was walking off into the stands.
"The courage and bravery the lads showed was second to none."
Wales coach Warren Gatland was also left "gutted" by the sending off, believing the incident denied his side the chance to compete effectively.
His side also lost prop Adam Jones to injury early on. He was replaced by Paul James at tight-head prop.
The International Rugby Board's rule on 'spear tackles' reads: "It is dangerous play to lift a player from the ground and drop or drive that player into the ground whilst that player's feet are still off the ground so that the player's head and/or upper body come into contact with the ground."
Ahead of the tournament a directive was also issued to referees relating to such tackles, which stated: "Foul play - high tackles, grabbing and twisting of the head and tip tackles to be emphasised, with referees to start at red and work backwards."
IRB recommended sanctions for such tackles start at bans of three weeks for offences considered to be at the lower end of the scale, six weeks for middle-level transgressions and end at 10 weeks or more for the worst 'spear tackles'.
But Warburton's sending off prompted instant condemnation among watching pundits, players and coaches.
Former Wales defence coach Clive Griffiths said: "That is the worst decision I've seen in my whole career."
South Africa's 1995 World Cup-winning captain Francois Pienaar said: "It is wrong, it is wrong. It was a dangerous tackle, yes, it was a penalty, yes.
"At best it was a yellow card, it was never a red card. It killed the game. In a World Cup semi-final, with the world watching, you have all that technology at your disposal, why not make that decision with the video referee?"
Wales flanker Martyn Williams, who was overlooked for the tournament in Warburton's favour, said: "He [referee Alain Rolland] has got to have more empathy for the game.
"If anything Sam has made too good a tackle. It is a ridiculous decision."
Former England captain Lawrence Dallaglio added: "He has lifted him up, realised his feet have gone up, and then let him go.
"This ref refereed the last World Cup final, he is supposed to be one of the best in the world. But he has made a terrible decision."
Warburton is the second Welshman to be sent off in a World Cup.
The first was Huw Richards, of Neath, against New Zealand in the 1987 semi-final.
Richards was dismissed for punching Gary Whetton in a game Wales lost 49-6, but they went on to earn third place after beating Australia in the play-off in Rotorua.
New Zealand and Australia face each other on Sunday in the second semi-final with Wales to face the losers.