Physics.SE is not a homework help site, it is not meant to be a resource to teach people physics, and it is not a resource intended to help right conceptual misconceptions in students. When it was originally created, this site was meant to be a place where active physicists doing active research could come and ask/answer questions. It was intended to be a resource for physicists to be able to find answers to common questions they may want to ask. Much like how SO is most useful to programmers that are trying to program.
Since its creation, the population of Physics.SE has opened the site up to be more available to a broader audience. No longer is it specifically catering to professional physicists. The site is now a place where you can ask a question about physics, its concepts and explanations, and get an answer. We do enjoy it when we end up teaching something, however that is still not the objective. The objective is to answer physics questions that are broadly useful to physicists. If we choose to answer a question that roughly teaches the physics behind an everyday phenomenon or introduces the basic concepts of some advanced physics concept, that is our prerogative. And we expect that these answers will be useful in some way to active physicists. They may refresh concepts and ideas for those who are teaching them, remind others of stuff they learned long ago, or help out when a physicist is starting to research in a new area/field.
If we choose to answer a question such that the layman would benefit most from it, that is our prerogative as well. We have chosen to extend the range of usefulness to include offering a conceptual understanding to students and non-physicists, but there is no mandate for us to do this. The site is still primarily by physicists and of physicists and for physicists.
Many homework-like questions and, in particular, check my work questions can and very well may be helpful to students. We don't care. Our goal is not to teach students or ensure that they learn; it is to help physicists. Check-my-work questions do not help physicists, they only waste the time of the answerer. The question may be able to be reformed such that it is asking about the concepts of a problem, and that could help a physicist in teaching a class or brushing up on their own understanding. Even if the mistake is a conceptual error, the question is still only specific to students.
Even though it is possible that some of these questions could be reworked into a form that we'd find acceptable, the general form of check-my-work questions is undesirable and off-topic to us. Therefore, we want to discourage all of these questions from being posed in the first place. When someone posts such a question, the likelihood is that they do not know what the problem is, whether it is math or conceptual. If they see that some check-my-work questions are allowed because of what the resolution was, then, not knowing what theirs may be, they will always post their question and we will continue getting questions that are just plain wrong for this site. If they see that no check-my-work questions are allowed, at best they will not post their question at all and perhaps on average they will rework their question to fit into what we might consider an acceptable format.
Nevertheless, you must still keep in mind that no check-my-work questions, no matter the difficulty the OP has, are in line with the spirit of this site. We exist to provide physicists with a database of helpful questions and answers. We choose to also provide physical insight and explanations into common questions for the layman and students because we are very nice people, not because we have to. Most homework help questions have no use to physicists and so we do not accept most of them. Where we choose to draw the line in how much we will cater to non-physicists is entirely up to the collective body of users on this site. You cannot say we are wrong for choosing not to accept these questions. We have no requirement to help students with homework-like problems, so if we decide that we will not do so, the only thing to say is "Have it your way" and move on.
Summary: We aren't here to help students, so there's no use in being upset when we choose not to.