Another thing I was thinking about is that in school, there's often a certain amount of emphasis on how to cite a book, article, or other piece of text. People don't necessarily learn or remember the details correctly, but I think it sticks in their minds that a longish text is something that ought to be attributed. (Granted, there are people who defy this.) Maybe with multimedia being easier to access, schools are teaching more about citing photos, diagrams, videos, song clips, etc., but this wasn't really even on the radar when I was in school--a total non-issue. So not as many people get that "catch" in their brain that says, "Hey, maybe I should say where I got this..." when looking at a photo.
These are all reasons why a normally polite person might simply not think of citing photos. Hopefully a polite reminder would be all it takes for them to rectify the situation, and be more observant in the future. Obviously there are other people who go around deliberately stealing stuff and taking credit for it, but I would like to think they're in the minority.