It only shows "Update and shut down" (which is a lie as it still will reboot) or "Update and reboot.

I think all this was a response to severely outdated Windows machines being infected with worms and what not. Microsoft got bad press for this and went (way) overboard with trying to force users to install updates as soon as released.

I wonder whether it has to do with the actual installation being in progress in the background. There is probably a time window in some updates where an interruption leaves the OS in a bad state.