It probably makes sense for most pack formats to untaint. But I agree that it would be better if the "a", "A", and "Z" formats would not untaint. My guess is that it was an intentional features due to the other formats. But making an exception for those few makes perfect sense (or, more accurately, make it so feeding a tainted value as input to an a/A/Z format should cause the output of pack to also be tainted).