Dead ball fouls are considered serious enough that it doesn't matter what happened during the play. Let's say a defender picks off a pass (but was actually slightly out of bounds which was only seen later on review) and the returning team holds while the defense returns it for a touchdown. After the play, an offensive lineman puts a cheap shot on the returner and draws an unsportsmanlike conduct foul. Upon review, the touchdown and interception are negated. Assessing the holding penalty makes no sense because it was a live ball foul that occurred in the context of the play and there's nowhere to really assess the penalty from. The unsportsmanlike conduct penalty stands even though the lineman probably wouldn't have delivered that cheap shot if it were simply an incomplete pass. However, it doesn't really matter why the offending player delivered the hit. It can be assessed as a fifteen yard penalty after the previous play completed, whether it's a pick six, an interception with a return because by a holding call, or an incomplete pass.
