It makes sense to me. The one person in the car was driving. They didn't want SP to jump out until they were ready for her to do it. They also didn't want to have to get out of the vehicle and come around to let her out because that could have exposed them to passing motorists.
So let's envision SP being in the right rear. She has one arm/hand tied to the waist chain. Let's say it's the left. The other limb, the right one, is chained to something heavy/stationary inside the vehicle - like underneath a seat.
When the perp is at the drop-off point, they possibly point a gun at SP and tell her to put her right hand on the door handle and keep it there. They then cut the chain from the seat with a bolt cutter and order her out.
At that point she is of no risk to the perp's personal safety. Her left arm is secured to the waist chain, so she can't lunge forward to the front to try to grab hair or rake nails across skin to get DNA. Keep in mind she also still has the bag over her head.
Her only move is to open the right rear door with her right hand and jump out. As soon as her feet hit the ground, the perp hits the gas hard and the movement forward causes the door to shut.
That is pretty much how I would do it if I were solely in charge of letting her go but wanted to protect myself in the process.