How can I get my parents to stop being so obsessive about my grades?

I'm a junior in highschool who's in 5 AP classes and a National Merit Semifinalist and my parents freak out over every little thing that happens in school. My mom checks my grades on the internet multiple times per day and if I have a B in anything she hides my laptop (which I do all my school work on) from me until it is raised to an A, even if there is no way that I can raise it. She pretty much removes everything enjoyable from my life until I get all As. I really don't understand why she even cares because I have a 4.2 GPA, so even if I was to get a C in every class for the rest of highschool I could still get a full ride to the college I want to go to because of National Merit. However, I have never gotten a C in any class, so unless I somehow start doing horribly in all my classes it is literally impossible for me to get less than a 4.0 GPA (weighted). Please don't interpret this as me saying I don't care about my grades; I really do, which is what my parents don't understand when I say it doesn't matter if I get a B because it won't lower my GPA at all.

So this semester I'm getting two B's; one because my history teacher gives us pop quizzes over material we haven't covered; the other because I had an A in the class all semester and did bad enough on the semester test that it lowered my grade to a B. Because of these grades I will be subjected to verbal degradation and grounded from my laptop for all of Winter Break. The basic purpose of this question is to find out how I can convince my parents to be reasonable and listen to logic, if that's even possible for them. I understand that they care about me and want me to do the best possible, but they can't understand that getting a B has no impact on my GPA, and therefore I should not be punished for getting one. It's really depressing when your parents outright say that they are disgusted with you and that your grades are unnacceptable when you did your best. (It's also ironic considering that my mom is essentially making minimum wage and my dad works as a supervisor on a construction site because he has no college degree)

Any advice on what to do in this situation would be much appreciated! (Please no comments about how I'm being arrogant or unreasonable, after all I am the one saving my parents 40,000 dollars on college)