I think that the purpose behind this exemption to determining FC status is to save us some headaches. When the player makes a catch and ends up with one foot FC and one foot BC, we are treating it as if both feet landed simultaneously.
Imagine trying to determine if you should call a violation because the FC foot landed 0.1 seconds or less before the BC foot. Or what if the defensive player was fumbling the ball when the first foot hit and then gained control once the BC foot came down? We already have enough difficult judgement calls to make. This rule makes our lives easier... (maybe we should have more rules like this
)