They have books that tell you what is and what is not considered polite in other cultures. My favorite series is Culture Shock.
I would think the boys were told things like that before they came here, but teenagers don't think the same as adults, so maybe they just disregarded that info. I'd bet they weren't trying to be rude, but it would be nice for them to make the effort to find out what is considered rude in the culture they'll be spending time in.
To me, it doesn't seem like anyone would possibly think it would be ok to call someone else lazy and fat. Can you imagine what the German version of the CB would be like?