Basically because that 5% is saying the other 95% should never have the option for a device without third party App Stores. It is a small minority trying to dictate what options the majority has available.
No one is forcing anyone to install an app store. But this logic you are already being forced to switch to Android and install a third-party app store for applications that aren't allowed on the Apple App Store or Google Play. The reality is that you aren't being forced to do that today and won't be if you can install software without approval from Apple in iOS.