What are the social norms associated with stealing free coffee? Every morning, at the institution where I work, I attend a lecture series with on the order of 100 other students. The lecture hall is very central on campus, and large meetings are often held nearby -- almost right outside -- at the same time. Very frequently, tables equipt with refreshments accompany these meetings. Again very frequently, I will pour myself a cup of coffee as I go by.

I'm reasonably certain that this is unethical (*), but what surprises me is how socially deviant it appears. Of the approximately 100 students with whom I attend lectures, I do not see any of them making use of these resources on a regular basis. I've once or twice been harshly reprimanded by a conference-goer getting coffee at the same time as myself, and I've seen people sit down to work only about 20 meters away with coffee that they've licitly purchased, even if the meeting was clearly over and the refreshments were just waiting to be cleaned up. I once walked by a table just as it was being cleared and asked the staff member clearing it if it was okay for me to take something, and he was totally cool with it. I've never seen signs posted near the tables indicating that the refreshments are intended strictly for the attendees of the meeting, though I would guess that this is assumed to be understood. My only other source for views on this matter is PHD Comics, which has published an assortment of strips centered on the topic of free food.

I know that social norms probably vary geographically, maybe even by discipline, but I was wondering what others' experiences have been as regards stealing free food. As is -- I believe -- apparent, I consider it a relatively minor trespass, but other than the conference-goers who've yelled at me I've never heard the issue brought up explicitly. So, how do you think people judge the stealing of free food, and how have you formed these perceptions?

(*) I'm only "reasonably certain" because, as I understand it, the excesses from these meeting are frequently thrown out. As someone legitimately attending a conference, I've been instructed explicitly by the organizers to take excess food with me to avoid wasting it in this manner. I dislike waste in general, and I particularly abhor the waste of perfectly good food, especially coffee.

Note: I'm not 100% sure that this belongs on Academia Stack Exchange, but this is not a question about the ethics of stealing coffee; I am interested specifically in the attitudes of academics towards stealing free food. For instance, why do so few others seem to do so? People do little unethical things all the time -- arguably, speeding falls in this category -- but I seem almost alone in breaking this particular rule.
The big difference here is simply whether the meeting is over or not, i.e. whether it would have to be thrown away or not. If those who are supposed to consume it (for which, as others have pointed out, someone paid) clearly have no intent of doing so (because they left), then I think it is fine to consume it. 

In fact, in that case, you would even do them a favor because noone has to clean it up. But otherwise, it's stealing. Another indication for this is the group size: With about 100 students, there clearly wouldn't be enough if everyone took some coffee.