They said they had been made "scapegoats", claiming that the wider forest fire was sparked not by their barbecue but by some other source.
Molteni's lawyer, Ivana Anomali, said that fining the young men such a huge sum made no sense because they would never be able to pay the sum.
"What is the sense of imposing a sanction of €13.5 million each knowing that these two kids, who are still students, cannot pay it," she told La Stampa.
She said the matter would go to trial, at which the young men would likely appeal the fine and argue that the forest fire did not originate with their barbecue.
A forensic investigation by the forestry service indicated, however, that the fire started from the barbecue that the students had set up in the garden of Borghi's grandfather, who may be held legally liable for paying a portion of the fine.
The forestry service calculated the massive penalty by measuring how much land had been burnt and how much it was worth per square metre.
A prosecutor in the case said that the fine should act as a warning to others.
"We need to put pressure on people to take greater responsibility for the environment and to prevent the imprudent use of flammable materials," said chief prosecutor Nicola Piacente.
He said the fine was "unprecedented" and took into account the cost of deploying firefighters and water-bombing aircraft.
Attilio Fontana, the governor of Lombardy, the region where the fire took place, said the size of the penalty "gives an idea of the scale of the damage that was done".
"I hope that these kids will pay more attention next time," he said.
