Because it really is unnecessary. iOS doesn't run the same as Android. When the apps are "running" in the background, they really aren't running.
This doesn't matter.
The only reason you need to close an app manually is if it is frozen
If someone's happy to have a certain app use BAR but temporarily want to stop it (because it uses more battery), why does it bother you so much if that someone chooses to quit the app? Lot simpler than going into the settings, disabling, remembering to enabled later.
And it's besides the point. The point is that quitting apps has an effect in more cases these days. This isn't iOS 4.