Why might the author have chosen to incorporate such profanity as diction, whereas the novel was a banned book in the American classroom?

I don't think that Salinger was thinking about audience rejection when he wrote this. Now, of course, I never met Salinger but I know the guy hated publicity and was pretty much a hermit. Artists (I include authors) like Salinger who create brilliant stuff like Catcher in the Rye, generally do it to be true to their characters. They love their characters. Part of what makes Holden Caulfield so popular generation after generation is that he is real. His diction is part of who he is and, perhaps, we see a little of ourselves in him. Many famous works of literature have been band in America. What are the reasons for this? Well, that's a much longer discussion.