Probably best to ask mom and dad why they want this. Obviously they got this for a reason. Do you have younger siblings that they want to know about and since you are still on their plan you are coming along for the ride? That's the case with us, with a 19 year old away at college but we still have 16, 13 and 11 year old kids at home.
The 19 year old understands that if she wants us to pay for her service this is something she has to live with (but we have no reason to see where she is). Like others have said maybe it's time to get your own line and cut the cord.