The mall manager claims that teenagers congregating in large groups near stores have caused adult shoppers to spend less time shopping than before. In order to prevent a significant loss in overall mall sales, the manager says, merchants should discourage teenagers from forming groups near stores. The merchant responds by pointing out that the teenagers congregating in these groups actually spend a significant percentage of the amount of money spent in those stores.

The merchant doesn't necessarily disagree with anything the manager has said — he just points out another relevant factor the manager hasn't considered. Even if adults are spending less time shopping at the mall because of the teenagers, the teenagers themselves are spending a significant amount of money. And we don't know how the amount of money that would be spent by adults compares to the amount spent by the teenagers now.

So stopping teenagers from congregating might not help in achieving the mall's goal of avoiding a loss in overall sales. If anything, driving the teenagers away might itself cause a significant loss in sales, which would be the opposite effect to what the manager intends. So the merchant provides relevant additional information that raises questions about the effect of the manager's suggestion.