Strange!
Have you tried force stopping the app first? Turn off fastboot and reboot? I will admit I have never tried to disable something like "Internet", as you show, which is the built-in browser. I was able to disable every single thing I wanted to and tried, except Sprint Zone. I just looked at my "Internet" and it is not allowed to be disabled at the moment (although I didn't force stop it).
Perhaps there are other built-in apps that are considered critical and are not disablable? Certainly what you say about "every" preinstalled app is not true, I just went through many and only a few others had the "disable" button greyed out.