When do you think Brad made his final decision to go with Emily (assuming Emily is F1)? I think it's pretty clear from the last episode that he was going to keep Ashley up until that last second. He even said "things change so quickly." I'm not sure how much he was considering Chantal at that point or even before this rose ceremony (back at the previous RC when he said goodbye to Shawntel.)
It just seems a little mean to keep Chantal there til the very end when she exhibited the most feelings for him and told him pretty early on that she was falling in love. The decision to let Ashley go seemed pretty impulsive.