The rule’s pretty clear, even if the enforcement of it is not. You can be in an offside position, but once you infer with play, the whistle should go. Whether that be by touching the ball or influencing another’s decisions, you’re offside once your team’s gained an advantage from your position.
The only question, above, is whether Mike Magee inferred with play, because he was certainly in an offside position. Implicitly, according to referee Kevin Stott, Red Bulls goalkeeper Luis Robles shouldn’t have been thrown off my Magee’s charge toward him. As such, Shipp’s goal was allowed to stand, giving visiting (and winless) Chicago a 1-0 lead.
It was the wrong call. Just watch Robles’s feet in the video, above. Notice how the New York keeper hesitates in response to Magee. Instead of continuing his charge for the ball, he stops and prepares to make himself big ahead of a possible header. Then, seemingly assuming Magee would be ruled offside, Robles’s jaw slackens when the goal is allowed to stand.
Blown offside calls are frequent enough that we’ve ceased being surprised when they happen. Normally, it’s a tough call, judging where a sprinting player is at a given point in time relative to another man. This one, however, shouldn’t have been a difficult decision. Even in real-time, Robles looked like he was thrown off my Magee’s run.