Well if these apps are actually running in the background, maybe that accounts for a lot of the battery drain issues I've been experiencing with 8.1 Preview. I'm still seeing my battery drain by large amounts when it's just sitting there idle.
I think the reason behind this behavior, if I remember correctly, is for a less confusing experience for new users, who don't necessarily expect an app to close when tapping the back button. It's a little annoying at first, but at least for me, not too hard to get used to.
And yeah, this will be the default behavior on Windows Phone 8.1.