If a safety issue as stated why charge more to allow a heavier bag through? If 70 lb was at the limit and a hard red line above which baggage cannot be processed, then that should be absolute as opposed to charging a fee for a heavier bag. I have been in airports in Europe where it is absolute and you cannot have more than 32kg even for a fee; in one airport there agents told me once that the conveyor belt is programmed and won't move if the bag weighs more than 32kg, so i had to remove stuff from by bag before it would move.