The ability to run arbitrary software on a computer is not required to call it bug-free. You can't safely run whatever software you want on the computers in your car, for example.
I replied by the logic that forfeiting guarantee is unreasonable since sideloaded apps can only break as much as existing apps can.
There are good reasons to disallow any third party applications on some platforms like cars, but apple allows it and they only have a quick look at applications. The real security is in their sandbox/API.
