Comment 28 for bug 1033226

Maybe this will clarify the situation: Let's say we turned this into a configurable behavior. What would the configuration option say? "Force user to reboot after updates - OFF/ON"

Can you imagine anyone purposely setting that option to ON for their own system? Of course not. Even people who take security updates very seriously might have something they want to do before they restart, and they definitely have the skill to click the restart option in the normal menu on their own.

The current behavior is ridiculous. Please see my comment on
https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/917452

If you want my recommendation: Give an informational notice dialog after installing security updates that require a reboot to take effect. It could say something like, 'Critical security updates have been installed, but they won't take effect until you reboot'. It should have a single button on it - 'OK'.