From Apple:
You can turn Automatic Downloads on or off over your cellular network on an iPhone or iPad Wi-Fi + 3G. Go to Settings > Store and turn Cellular Data on or off. Cellular Data downloads are limited to a file size of 50 MB or smaller.
Therefore, as a system update is larger than 50MB, it means, as I originally indicated, it will not download over your mobile connection, irrespective of any settings.
This also explains why Apple only provide a means to only disable app update auto-downloads..... the auto-download of system updates is effectively disabled by default.
For the record I only accused you of "apple bashing" because of your last paragraph in your original post about "apple knows best" etc..... that sort of phrasing generally only comes from the bashing community.... but I'm happy to give you the benefit of the doubt that your post was written in frustration over a misguided concern ! Virtual handshake and move on ?