From what I understand the 5S does not support low light boost mode. I don't have any insight into Apple's decision on this, but I'm guessing the improvements made to the camera/sensor on the 5S were good enough that they felt low light boost was not needed on that device.
So, what you're seeing is not an error. It's simply that some devices support that feature (the iPhone 5), and some do not (the iPhone 5S).
I'd post a list of compatible devices, but I don't have newer iPod Touches and some iPads to be able to test it. I'd guess the iPhone 5c does support it since that device is basically a reskinned iPhone 5, but again I don't have one to test.