It wasn't just the bears who were a little nervous. The fishermen had never been in this situation before either. All they knew about bears was to stay out of their way! Keeping hold of the bears wasn't an option.
They were clearly agitated, and as they recovered from the cold, they might become aggressive. Handing them back to the mother might be even more dangerous, though. It would mean heading straight for her. She could easily interpret that move as a sign of aggression!