It's touch sensitive, so like whenever he's dead or knocked back it won't kill anyone who's just standing around, or maybe GL didn't want to have to make special effects of the floor getting burned by the lightsaber. And also what if the guys got knocked back, and the lightsaber flew up and was still on then killed him.And like on the Obi-Wan vs Darth Maul scene, what if Obi-Wan accidentaly stepped on the lightsaber that Qui-Gon dropped. and when he had to use force pull to get Qui-Gon's lightsaber, what if it was on and it stabbed him.