It doesn't apply on uphill finishes where there's no reasonable assumption that the rider would have got home, without the mechanical, in the same time as the group. I suppose it is to avoid, as would have happened today, having the GC turned upside down because of chance, albeit chance with 3 km to go rather than 3.1. And to discourage continued fighting for places from other riders when the sprinters are getting ready. Though since Roglic made the (unofficially) uphill nature of the finish today work to his advantage it's not always clear cut.
It doesn't apply on uphill finishes where there's no reasonable assumption that the rider would have got home, without the mechanical, in the same time as the group. I suppose it is to avoid, as would have happened today, having the GC turned upside down because of chance, albeit chance with 3 km to go rather than 3.1. And to discourage continued fighting for places from other riders when the sprinters are getting ready. Though since Roglic made the (unofficially) uphill nature of the finish today work to his advantage it's not always clear cut.