The ball rebounded to Lamah off Baggies goalkeeper Ben Foster and defender Gareth McAuley.
Laudrup said: "You can never be offside if you receive the ball from an opponent. That's the rules of football. It has nothing to do with good or bad decisions.
"It is a huge mistake because it is the rules of football. The linesman is standing there. He can see it."
But Swans defender Ashley Williams was more concerned about his side's performance.
"Looking at our performance as a team we could have done better. The incident at the end has gone, there's no point in talking about it," said the big centre-half.
"Some of the lads spoke to the linesman, I think he admitted it was a mistake but I'm not too sure. I've seen it on video and it's a clear goal.
"It's gone, we need to talk about our performances. When we scored, we stopped playing for some reason. We stopped attacking them, they're a good side and a team like that you have to finish off.
"That's the frustrating thing about this game. We did brilliantly last week against Newcastle. This time I felt we took our foot off the pedal and let them back into it.
"As a dressing room we're frustrated. As a team we have to be better than that. We've got to continue to push ourselves, it's not about being safe. This was a game we could have won, we let it slip. We'll be disappointed if we don't finish in the top 10."