It has to do with how they spot the ball for the next play. If any part of the ball is in the end zone (and the lines are part of the end zone), then they would be spotting the ball in the end zone for the next play. Ridiculous, no? Ditto for safeties -- if any part of the ball remains in the end zone, it's a safety.
"Ridiculous, no?"
Totally ridiculous. Alas, football is a game of possession, and now I see why the implementation of this rule is needed. Thanks.