Actually, it is. The difference is those kinds of activities aren't specifically identified. Certainly DUI is a prime example, because the same laws apply to alcohol as do to medications, legal or otherwise as well as other chemical substances. For example, if you work in a body shop and were exposed to solvents that caused dizziness and you drove, you could be cited with a DUI violation, even though you were 'sober'.
In any case, if you take your eyes off the road and cause an accident, many states will cite you. Many years ago, I was sitting at a stop sign and was rear ended by a woman who had reached around to pick up her child's bottle while still moving. It was a minor fender bender with no injuries, but she was cited with a moving violation.
I think the point is that cell phone distraction as a cause of accidents is reaching enough statistical traction as to warrant it being specifically listed so that you may be stopped and cited for it, just as seat belt use (or lack thereof) could only be cited if you were stopped for another offense.
I commute 60+ miles a day on the highway and idiots people who have cell phones glued to their ears for miles scare the crap out of me because you can tell they are not paying attention by the way they cross lines, slow down in the passing lanes and cut people off when they realize where they are.
