Hey, an actually interesting topic.
Probably a bit of both as you suggested. On the other hand, I know some Chinese/Chinese American teens who don't really care about the politics of the situation. Interesting to see to what extent pop culture, rather than nationalistic propoganda, influences either side. But I'm sure the Chinese government wouldn't be too thrilled with their media showing Japanese pop culture entertainment anyway.
It might conceivably also be a bit of the second factor that you mentioned, too. Most J"Pop" discussions these days concentrate on a certain genre of Japanese music which has limited appeal. Some discussions also get kind of eccentric, too (to speak kindly- I'm sure I'll still get harassed, but that's the problem with overzealous fans, with whom I've long, long given up). With such a limited view, it's not hard to see why there'd be interest elsewhere. OR, to be fair to them, maybe fans of jpop are actually broadening their horizons to be less exclusionary in their tastes, which is a Good Thing.
Interestingly, there have been some Japanese artists touring in China recently. One I can think right off the bat would be L'arc en Ciel's AWAKE Asia Tour. I think there have been others, but I forget which.