"If I go to the Software center and click "Install"" -- that is why it triggers the reboot prompt. You, the human, are electing to install it now. Once the install is complete, then the prompt occurs, and no, it won't 'just wait until the weekend'. You the human have spoken!
If you want CM to "just do things during the Service Window', then you have to let it do so. Don't go in and manually trigger it to install from Software Center. When you do that (or trigger it remotely, which can be done with powershell commands if you so desire), you've bypassed the required deadlines, and the Service Window things around "what to do if the deadline is before my service window". As an admin, you manually bypassed all of that by clicking on it in Software Center.