Because adults sometimes watch children's programming too (yes, really), and as far as they can tell, that's what's going on.
But to be honest, maybe this is an opportunity to create some kind of maturity level standard, where content shouldn't be above a set threshold, regardless of source. On YouTube For Kids, it would be lower than regular YouTube, which would be lower than RedTube. Advertising would be compelled to build in functionality that checks a page's threshold and serves ads that match.
I'm not saying I agree with the idea, just putting it out there.
That's what every advertising platform does...
It doesn't matter that some adults would watch also. You would never in a million years see an ad for The Wire while watching Sesame Street or Mr. Rogers, etc. Pretty obvious why not...
The same standard should be applied to "new" media.