Why do iTunes parental controls only affect content in the iTunes store and not the movies in my collection downloaded on my mac?

I'd like to be able to use parental controls in iTunes to stop my kids from accidently clicking on Rated R movies but I can't seem to get that to work. You'd think this is a very BASIC feature that a company like Apple could incorporate but that doesn't appear to be the case. I've gone to the iTunes preferences->parental Controls and ive restricted anything greater than a RATED G movie yet ALL MOVIES STILL PLAY regardless of their rating. It appears the parental controls only affects the content they can view or purchase from the itunes store. Am i missing something here? Just to recap.. I have about 100 movies in my itunes library and I want to restrict anything over a G rating. How can I do that? Surely there is a way to do this on the computer for movies that are downloaded on the pc and not in the cloud. Pointless to have the parental controls if you ask me.