You don't see the irony in this? If your parents are dropping $56k YEARLY on you, YOU ARE NOT AN ADULT. Why should his parents "let me do things my own way" if they are dumping THAT kind of money on me? I don't give an auto-mechanic $1000 and tell him to do whatever the heck he wants with it. That money is for a reason. And $56k/yearly ought to buy you a freakin' 4.0! No excuses!
Partying and dating = troublemaker? Sheesh...were your parents Adolf Hitler and Eva Braun?
I understand where you are coming from. If you're parents have to pay for you, technically you're not an adult. Though I think there is a difference is being 30, living in your parents garage, etc... and being in school full-time and trying to get a degree (when you're like 18).
I don't personally know the OP, so I can't comment on his maturity or whether the fact that he is an adult yet or not. But I think in our society, there is a definite difference between adult and independent due to the fact that everyone goes to school well into their 20s. So you can be an adult, while still technically being dependent on your parents, to a certain degree.
I also don't think the OP is trying to do things completely his own way (ie run off, party like no other, and let his grades tank). I think he's just asking for a little breathing room and not be hit over the head with the fact, "I'm paying for you, you will do what I want". I disagree with your mechanic analogy in the sense that the people who are providing the OP with money are his parents, not an employer. If I give an employer money, yes I want him to get the job done. If I support my child through school, I expect them to try their best and try to become successful, it doesn't give me the right, however, to demand that he become a certain career or unreasonable things. (Though here, I don't think that showing your grades to your parents is unreasonable, as I've stated to the OP) I do think that refusing to speak to him after a B for several days or SEVERE guilt-tripping is wrong however.
Parents I feel who expect really certain things for their kids (e.g. you have to become a pharmacist!!) and not just general expectations (do good, be a good human being, go to college, etc) never really wanted kids, just robots they could mold.
As for my sister, suitup is right. I did not elaborate. Pretty much (without the need to go into specific detail), she was all out in high school (and not in the good way), my parents just did not know what to do. But she's more responsible and mature now, so it's all good.