Talking on a cellphone is radically different than checking text messages. The latter requires you to take your eyes off the road. Cellphone usage makes people more likely to get in accidents, including hands-free cellphones. So the issue is distraction/engagement, at least in part. But listening to the radio, or talking with a fellow passenger (especially if they're in the backseat) may have the same effects. You're making a pretty big leap here.