They are usually Japanese or Korean from what I know and as L.A Guy pointed they are rarely Chinese. The Chinese often snicker at them, I don't know why I never ventured to ask.
I might have a little bit of insight here, Japanese fashion and trends are kind of a guilty pleasure of mine. I don't participate in them (being in the U.S. it, would be rather odd) but I keep track of the latest and most bizzare as sort of a hobby. My best guess is that the Japanese youths of which you speak, were either born in, or lived extensivly in Japan, or were raised in a very traditional Japanese household. Because of the many differences between Western and Japanese culture, they are trying to fit in better in western society by easily identifying themselves within currently popularized stereotypes. Conformity is very important to the Japanese. Individualism at one point in time was considered abberant behavior. Your greatest goal in life was to be a cog in the corporate machine. Now that the bubble economy has collapsed, the western business model is starting to gain a foothold, and the individual ideal is just starting to be discovered. The Japanese and Korean societies don't really have a strong individualistic identity like we do here in the West, so they tend to fall back on stereotypes and distorted western media perceptions to try and - for lack of a better term - seek their own individuality by emulating the things they feel best reflect their general attitudes and beliefs (I.E. - I've been led to believe that Americans all watch MTV. Almost all of the people on MTV are rappers, who are tough. I want to be tough, therefore I'm going to wear FUBU. Therefore I'm an American."). Sounds strange, I know... but I assure you, thought processes go on like that every day in the warped minds of the Japanese teenager. Its closest western counterpart that I can really categorize is the Abercrombie & Fitch "movement". For some reason millions of people age 12-25+ find it neccesary to shop for and wear unbearably bland clothing. A&F, Eddie Bauer, GAP, etc. Does anyone remember a time when you couldn't tell what brand of grey, hooded sweatshirt someone was wearing two blocks away? The logo seems to be the major selling point now, as far as I can tell. It must be because everyone else does it. Someone else can try to explain that one. I'm getting depressed just thinking about it. And I think the Chinese are laughing because they hate us Japanese for WW2. They enjoy us being behind the curve in anything, they don't care what it is. And they are laughing at the Koreans who think they are Japanese. But the Japanese laugh at everyone, it's just that nobody can see us laughing because we have no outward emotions. Duh.