I agree, they write them basically because they can and to sell more products. I have never seen or heard of a virus made for PPCs either, but if your looking for something free, Flexilis advertizes that it does this: Keep your mobile device and data safe from loss, theft, hackers, viruses, malware, and spam.
I tried it for a bit, but drifted away. It is in beta and free, I am not sure if they will charge eventually, but it is free for now. You can find it here:
https://beta.flexilis.com/