There's only one reason that you build an obvious vulnerability into a system - to keep it from getting out of control. I would guess that to get close enough to mess with those controls (without Vader being able to stop you), you'd have to be pretty powerful. After the events of Revenge of the Sith, there were not many beings left in the universe that powerful aside from the Emperor.

In short, I'd say it was a reminder to Vader that the Emperor could turn him off with the press of a toggle switch if he get out of control.

There's only one reason that you build an obvious vulnerability into a system - to keep it from getting out of control. I would guess that to get close enough to mess with those controls (without Vader being able to stop you), you'd have to be pretty powerful. After the events of Revenge of the Sith, there were not many beings left in the universe that powerful aside from the Emperor.

In short, I'd say it was a reminder to Vader that the Emperor could turn him off with the press of a toggle switch if he get out of control.

EDIT: So how did a Padawan (Antinnis Tremayne) achieve it? Vader allowed it, seeing who could think beyond a simple direct attack...the exercise was a test of lateral thinking.