If you're driving in stop-and-go traffic, texting using your cell phone isn't quite as bad as if you're barreling down the highway at 70, 80, 90 or more miles per hour while texting.
As for differentiating passengers and drivers ... tethering is one possibility. Not electronic tethering, but literal physical tethers that connect the phone to the car and are short enough and positioned carefully enough (for the front seat, in the far corner of the dashboard from the driver above the glove compartment) where it will allow passengers to bring the phone to their ears but will not allow drivers to do so. Any phone that is physically tethered can make a call via the car's antenna; phones that aren't are blocked.
Now sure, drivers could probably try to attach extender cables to allow them to text while driving. And if a police officer sees an extender, they can pull the driver over, confiscate the cable (and possibly the phone), and fine the offender.