The fishermen, who had all encountered bears before, knew there was more to worry about than just the cubs. The mother was still in the water near the island. Although she'd seemingly abandoned her cubs, there was still a risk that her protective instincts would kick in.
The fishermen didn't want to come under attack from an angry adult bear. This rescue mission needed to be enacted with precise timing. They had to save the cubs without inviting the ire of the mother.