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I explained it in another thread, but I'll repeat myself again.
A massive portion of the Pokemon fandom is composed of the Japanese, most of which are children ranging from ages 6 - 14. You'll never understand how popular it is over there until you go to Japan yourself and see the endless hordes of Jap kids lining up for cheap Pokemerch.
Due to this, there is a considerably larger chance of you coming across a selfish jap kid who's too young, immature or inconsiderate to realise he shouldn't put up shit mons, than an older or more sensible jap, or a western kid who is equally as bad as the japs.
It's not that the japs are worse, it's simply because the probability of receiving a shitmon from a jap is higher than any other simply because they're SO MANY of the bastards. Don't forget that they'll be japs out there who are equally pissed off as we are because some kid in the neighbouring prefecture sent him a niggersby.
If you did 100 wondertrades, 50 of them with japs and 50 with other races, the ratio of shitmons to other Pokemon would be the same, but if you wondertrade with 100 randoms, you'll encounter more japs and thus the chances of recieving a shitmon is highly than in a controlled environment.