Choosing categories for posts

Imagine I have the following categories:

Music (parent)
> Rock
> Hip Hop
> Classical

And then I create a new post for rock music, now I could select both Music and Rock or I could just select Rock. Why is this so? Why does WordPress not auto add the Music category to my post if I have selected Rock which is a child category?

The problem this causes is that if more than one user on a blog creates an article and one uses both Music and Rock categories and the other just selects Rock it will create havoc in terms of consistency.

Can someone clear up these issues and explain more about this and what the difference would be between choosing both parent and child or just child.

Thanks