Uhm, yes, that, precisely. Most of Android is on a level playing field - core framework services get elevated permissions under the system group but they're still apps. If you let the user uninstall them, all hell will break loose. It's like removing the svchost.exe process in Windows. Or removing /bin/sh in Linux. The rest just doesn't work without it. Things like Contacts, Calendar storage, Bluetooth, Play Store, Google Backup, etc, etc, are all in /system/app, where manufacturers also put their apps.

If you ask me, manufacturers should be prohibited from adding to /system/app but that's not how the Android licensing works. So, we get un-uninstallable apps (vote with your wallet - buy only Nexus experience!).