First, Windows 8 allows you to run whatever desktop apps you want, including third party antivirus software of your choice that have full access to the system.
Second, I haven't seen your argument made for iOS and Chromebooks which are much more locked down than Windows 8. Though one could argue that Chromebook doesn't need to have malware since everything is helpfully uploaded to the cloud.
If the third party antivirus software need to be approved by MS, then I can't run the software of my choice.
It might had been worth mentioned, I am not the first person to talk about liability issues regarding lockdown. I first heard it in a talk that described the iPhone.
Second, I haven't seen your argument made for iOS and Chromebooks which are much more locked down than Windows 8. Though one could argue that Chromebook doesn't need to have malware since everything is helpfully uploaded to the cloud.