It's not vague at all if you think about it the right way.
The rule (from the policies) is: "No Google ad may be placed on pages published specifically for the purpose of showing ads, whether or not the page content is relevant."
Note that it says PAGES not WEBSITES. I'm pointing that out because what I think that means is that you can't have a popup page or other page that has ONLY Adsense ads on it (without any other content). Some will argue this, but, I think that's what that means because there is no way that anyone can determine if a website is being published "specifically for the purpose of showing ads".