It doesn't mean they don't know. Your parents know it well, and they're just ignoring it. If they warn you, it might hurt you. They believe in you that you won't use it bad, and they'll keep an eye over you watching your activities. When they find something wrong is taking place with you, they'll warn you and restrict not to take it anymore.

Note: It is always good to inform your parents, they might scold. Just take it positive and make them understand what you do is good.