I think there are as many motivations for this sort of thing as people who do it. My guess is whoever this was had a carrel in that part of the building and when they left they had to "get rid of the evidence." I am heartened that we as a profession are being more open about thefts so that, at least for the rare and valuable things, we have a chance at getting the items back. For things like this, however, we just try to replace what we can through replacing the pages or getting a new copy.