Brock Goes Berserk!!!

Riddle: The Mills family had traveled to their vacation cabin which was next to a large, beautiful lake in the mountains. Mr. and Mrs. Mills and their three children, Brock (age 6), Cindy (age 7), and Susan (age 8) had come to the lake to relax for the weekend. They were all looking forward to taking a long, relaxing swim in the lake, and to try out the brand new boat Mr. Mills had just purchased for some family fun. As they were swimming near the middle of the lake, it dawned on Mr. Mills that their son Brock was not in the water with them. Suddenly, with no warning, the family heard motor sounds and saw their newly-purchased boat speeding directly toward them, accompanied by the maniacal laughter of their missing child, Brock. Again and again, Brock drove the boat directly at his vulnerable family, laughing insanely each time it passed near them. "I told Brock not to go near that boat unless I was with him," shouted Mr. Mills, "and now he's trying to kill us with it!" However, Mrs. Mills nonchalantly responded, "Oh, boys will be boys." Considering the circumstances, why was Mrs. Mills so cavalier concerning her son's disobedient and reckless behavior, and why did she seem unconcerned about her six-year-old driving a boat without supervision?