I don't expect Geroge Lucas's universe to be logically consistent, but simple deduction will show that lightsabers are not activated by the Force:
General Grievous can use lightsabers.
General Grievous cannot use the Force.
Therefore, creatures who cannot use the Force can use lightsabers.
It follows that lightsabers are activated by something else besides the Force (otherwise, General Grievous would have fought Obi-Wan with four inactive lightsabers), and a switch would be the most obvious explanation.