Because Tesla is assuming responsibility (that is, guaranteeing fitness for purpose, and all the ramifications that come with that) for the car being able to drive you under the same circumstances as any other private person driving. They would be taking on much more (legal, insurance, etc.) risk if they were to allow the car to drive as a passenger carrying, fare accepting, commercial entity, therefore it seems fair that they require you to use their managed system to (I assume) mitigate and/or manage that risk. I would guess that it would also be possible, eventually, to license the autopilot software for your own arbitrary commercial use.