Sep 11, 2012 - 03:44 PM
It's not clear to me what level/lesson you're at, but you're probably going to find a lot more of these "to de, or not to de" examples. They'll keep telling you that if you use de, it won't be wrong. But then will often leave it out. Bit by bit, you'll get more guidance, but it remains pretty "squishy". I'm half way through level 3, and they've mentioned the idea that if you can't actually "own" something (like an idea, or a mother in law), then you probably won't use "de". It also seems that "de" is often dropped when it's "obvious" that the word before it is an adjective, and nationalities seem to fall squarely into that category.
Sorry, this doesn't answer your question, but I think that might be because there isn't really an absolute rule in a grammatical sense, only that the Chinese drop it when it's obvious, and it's not clear to us when it's obvious...!
FWIW, on my trip to China, I used "Yingu de caidan" probably twice a day for ten days, and never got so much as a raised eyebrow. I did get a "bu you" followed by a menu with pictures a couple of times, but hey, that works too :)