The same would probably happen if they saw the kid with the Korean equivalent of this message on his T-shirt.
There's a language gap involved, to the extent that the kids themselves--much less the parents--don't know they're wearing anything rude (presumably in the same vein, one sees young men wearing "Miss Universe" t-shirts).
Perhaps there aren't any T-shirts with rude Korean text in the stores, and so parents don't have any reason to be suspicious of the English ones. The real question is--who's delivering these "U BRING THE ASS" and "I'm not a terrorist--Don't arrest me" to children's clothing stores? Also, are Korean citizens going to notice one day that the foreigners are laughing in all the wrong places?