The reason for that, in the US at least, is that "for white people" is implied by default. Everything is always for white people. So when you go and spell it out explicitly, "X for white people" ends up having a connotation of "X for white people only."
What poeron is getting at is there can be two meanings of "X for Y". While "X for kids" doesn't imply adults can't watch, "X for adults" implies kids can't watch.
