Why does Android require you to revert back to factory version in order to disable a system app?

In Android, you can go to the App info page and do things like see how much data storage it's using, force stop the app, and uninstall it. If the app isn't installed via the Play Store or APK (i.e., it's installed in the /system partition), the Uninstall button is replaced by the Disable button. Why does the Disable button require the app to be rolled back to the factory version before it will disable it?