Well parents are naturally protective , even in little things. And i can say, maybe they thinking you're doing illegal because they didn't know what kind or where the money really came from. Some bitcoin user are also hackers maybe they also heard things about that, and they want u to avoid getting involve.
But as long as you prove yourself , that you're doing fine and good just literally explain it always if needed but don't get so much argue about it , stay on being responsible child as you can. They just being protective if still insisting not too.