The intent is obvious. As stated by others, high school officials want to preserve the fantasy that athletes will restrict training to a small window of time while exploring the world, studying and enriching themselves in other ways during the "off-season."
This rule was developed in response to parents who complained that a high-school coach ruined their summer by enforcing practice schedules. I'm sure a coach even dared to kick athletes off a team for not practicing in the summer. And now you have a blanket rule.
This should work to your advantage. Any coach worth his salt can lay out a summer training plan in advance. If the coach doesn't, you have many more resources at your disposal than my generation did. And I'm going to assume that if you're posting here, you're more likely to stick to a summer training schedule than kids who need a daily practice to keep them in line.