Several commenters made variants of this argument: The government often warns people about enforcement of the law — e.g., by putting up signs telling people that the speed limit is photo enforced or radar enforced in a particular area. Likewise, people often laudably scare off criminals by telling them the police are coming. What’s the difference between that and warning someone to slow down because they see a police car?
I think there’s an important functional difference here. Speed limit enforcement signs genuinely tend to get people to drive within the law, at least so long as they are in that area. Scaring off a criminal tends to interrupt a crime, or prevent it from taking place. Headlight-flashing, on the other hand, gets people to abide by the law just for a few seconds, and in exchange makes it easier for people to evade punishment for their failure to abide by the law for all the rest of the time they drive in the area.
There’s also a difference in purpose. In the photo enforcement sign example and the scaring off the criminal example, the purpose of the warning is to prevent the crime. In the headlight-flashing example, the purpose of the warning is generally just to prevent apprehension.
Now what about the lookout, or for that matter the nonconfederate who happens to see the police coming and — without prearrangement — tells a drug dealer or graffiti painter to run off? There, the crime is indeed interrupted, and not just postponed by a few seconds, so the analysis might be different. Nonetheless, it seems to me that there is still a difference in purpose between someone whose purpose is to prevent the crime (and who may actually regret that the criminal will get off scot free) and someone whose purpose is to prevent the apprehension of a criminal. And obstruction of justice laws do indeed usually focus on the purpose, distinguishing those whose purpose it is to prevent apprehension from others.
Finally, some people pointed out that lookouts are often guilty of conspiracy, because they have a preexisting arrangement to serve as lookouts. But even people who warn that the police are coming, with the purpose of preventing apprehension, without any such earlier agreement can be guilty of obstruction of justice, and I think rightly so.