> Because our society believes in body autonomy, to the point that it won't use your body's organs to save other human beings, without your express consent.
> Your body is your property, and it takes a damn good reason for someone else to access your property, contrary to your will.
The way I see it, we're a guest on the planet, and after we are done for, there's a few things we leave behind: our direct ancestors (family), our indirect ancestors (mankind) and our planet as a whole (Earth with its inhabitants) many of whom are important for the planet's ecosystem. We should ensure that after we are done for, these living organisms survive and lead a happy and healthy life.
Now, I'm of the opinion that our justice system is relevant for society. It allows Earth's inhabitants to lead a happy and healthy life. This is particularly true for crimes such as murder/killing which have a deep impact on society. So if this heavy crime can be solved using the help of a deceased one, I'm all for that. Even if it involves force or naysayers.