I think the "by product" is also a way of people feeling better about it. In my mind, there is no difference between any product made harming animals, whether that be leather, fur or marine animals (for petrol-based fake fur and fake leather).
The only difference is that some fur animals, just like some farm animals, are treated better, and others are treated dreadfully.
Everything we use has some cost to animal life/suffering, and some products also cause harm and suffering to humans.
We educate ourselves and we make our choices about how much cruelty we will tolerate in the products we choose to use.
Perhaps fur is just a convenient scapegoat to make people feel good whilst ignoring the animal cruelty inherent in the products they choose to wear? Very few things are a "need", we won't die if we don't have that fur coat, fake or real. We don't die if we don't have that leather item, fake or otherwise. It's all about choice, but then, most of us choose not to live a very low-footprint consumer lifestyle. I know I like my comforts and spend most of my time in a "happy bubble" of ignorance about the cruelty of the dairy trade, the leather trade, the petro chemical industry, the slavery involved in chocolate. . . . . I could go on and on but I'm retreating back to my bubble.