It literally says "Select the system you want to use to start up your computer" and then it will confirm your selection.
I don't think it's complicated at all...I think you're making it way more complicated than it actually is....
The way the interface is done, it's as if I needed to click the "Restart..." button to complete the changes. What's the difference between "Restart..." and "Target Disk Mode..." ? The Horizontal Rule makes me think that the "Target Disk Mode..." option is not linked to the HD selection. Will it restart my Mac on the disk I selected and then undo my selection, as opposed to the first button doing the change permanently ? This pane is flawed in so many ways...
Even if I were wrong about it, the "if it's not broken, don't fix it" magic sentence still wouldn't work with me. Would you rather sit on a 10-years-old preferences pane or start afresh ?