The reason behind this rule is that metal detecting is allowed only during the off season. Off season is the time of year when people are not using the beaches for summer activities. This keeps the metal detectorist and the tourists separated. This way, you can metal detect without disturbing other people, and the beaches will not be packed with people.  This makes it much more enjoyable for the detectorist. You will not have as many people to worry about when detecting, yet you will be able to find all the items people lost over the more popular summer months. Plus, it can get quite hot in the summer, and it can be miserable to metal detect in the extremely hot weather.  Try searching after rainstorms or after the snow melts in the springtime. These events can help move the soil around and uncover items that were buried deeper the previous year.