There is a way to use either "for" or "because of" in those examples, but they aren't quite interchangeable. I think this is subtle, but in the second two sentences, using "because of" requires changing sentence structure. The use of the word "for" in each of those cases is vague. It leaves room for the reader to infer the details. Why are they in prison for murder? Well, I assume they committed it. "His novels" could refer to the novels he owned, but I instead infer he wrote them. When you use "because of", the reader will expect those details in the sentence. So here is how I would write them: She did 15 years in prison because she committed murder. I didn't say anything because I feared offending them. Saying "she did 15 years in prison because of murder" reads to me as "murder exists, therefore she's in prison". That's not quite right, so I added the word committed to be clear. Did that help any? I feel this is a subtle difference that involves on reader expectations.