[NSFW] Why do Japanese trysts sound so different?

First, a Disclaimer:
I can’t link to safe examples, as Fluther does not allow links to porn. If you wish to listen to what I mean, you might want to subscribe to Web of Trust to get trustworthiness ratings on search results before clicking through to them. Also, do not follow advertising links on porn pages or do anything other than close any pop-up, pop-under, or tab windows they spawn. Surf safe: use protection and discretion.

Now the Question:
Is there some cultural imperative that Japanese women always play the role of submissive victim? I had noticed it in Japanese heterosexual porn. Their sex sounds are quite different from those of western women, Africans or even other Asians. Instead of moans and heavy breathing, they whimper and sometimes even sound as if they are crying or being hurt.

I thought maybe it was intended to make Japanese men feel powerful, providing them some sort of thrill of victory and dominance. Maybe it was intended to let the guy imagine his penis was monstrous and too much for her. But then I noticed that it seems true even in lesbian petting videos where there is clearly no pain involved. The girl playing the more submissive role whimpers instead of moaning her pleasure; while the aggressor just breathes heavily. This was interesting, so I decided to see how Japanese females are portrayed in porn when they are doing it to themselves. The do the same whimpering when masturbating. Any thoughts as to why?