Flaj,
First, I've added links to the FAQ and the Forum rules to my signature. In the rules/guidelines/ thread, there is an entry on inside information. Last post, I believe.
GM can make an official request to obtain posting information (usually the IP address from which you post, which is enough to track your post to a server, which, when combined with other information, like when you posted etc., can lead back to you). This information is used to determine if you work for GM, a GM subsidiary, a GM supplier, or GM affiliate of some sort.
Posting "proprietary" or "confidential" or "secret" information, i.e. information not readily available outside the business environment of the company, gets the attention of those that lurk sites looking for information security breaches. Information breaches are usually plugged, and the source of the information can be (and has been) tracked down. Discipline to the poster can result. This discipline can be moderate (a mark in personnel record) to severe (firing the individual).
A company like GM can also do other stuff - if the breach is bad enough, they can (not saying they will or if there is a precident, just saying it is possible) create a LOT of problems for a forum. The end result may be a neutering of or even shutting down a forum in the process of tracking down breaches.
Also, many times, especially for automotive enthusiast forums, the members WANT to be heard by the people that WORK AT THE COMPANY THAT BUILDS THE PRODUCT. This site and the solstice sister site have a lot of lurkers - many of them are legitimately GM employees that want to see the reaction of potential customers. IF (again, not saying it has happened, just saying it is possible) GM gets upset enough, they can just shut off employee access to individual sites. The end result is employees no longer would be able to peruse our opinions, posts, likes, dislikes, heated discussions, concerns... etc.
So, take a look at the forum rules, the links to the articles, and digest this verbose post.
Hope that helps.