Generally, this policy is largely driven by What should we advise when one disagrees with the premise of a question? (which is, paraphrased, don't answer if you disagree with the premise). Parenting tends to be more strict about questioning the premise than the average StackExchange site, but we also tend to get more emotional or philosophical issues than average.
Most of our users are parents and we're hard-wired to try to help kids make good choices... but he didn't ask whether it was a bad idea. And even if he had, that would be off-topic -- we aren't HighSchoolSuccess.SE.
The OP's question is How can I get my parents to accept that I'm only trying to pass (that I will, in fact, pass), and that I'd like to spend my time doing harder/better things? In essence, it's a question about how to reach an understanding with one's parents about an approach to life, and therefore saying only "you're wrong" is not answering the question that was asked. (In contrast, saying "this is a lot harder than you might think, but here are ways to justify it" can convey "you're wrong"...)
The question does have something of an XY problem (he thinks it's all about his parents [Y], but in reality there's an issue with his approach to school [X]), but that doesn't completely eliminate the fact that their communication as a family is having problems. And that Y is what Parenting.SE should focus on, since that is the parenting part of the equation. His parents already want to be able to have a conversation with him about changing his behavior, and helping enable that conversation is a very topical and worthwhile endeavor. In contrast, us convincing him independently will perhaps make him study a bit more, go to college, and get a more secure life, but will it teach it how to talk to his parents about contentious or sensitive problems?
It may seem like a weak argument to be pointing to rules about topicality and relevance, but I feel like it also supports the big picture: helping that parent-child relationship function better.