Why can't you shut off phone while battery pack is attached and charging phone?

Iphone without battery if off, always turns on when you plug it in. However if it is on when you plug it in, you can always shut it off after you plug it in.
With the new battery pack attached, and while on "battery power" if you turn your phone off, it shuts off, however after it is off for a few seconds, it powers back on.
Is there anyway to shut the phone off without having to remove it from the charger case?

Apple iPhone 7 Smart Battery Case

Apple iPhone 7 Smart Battery Case

Product No Longer Available

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