You're not legally driving if your car is in park.
Umm, Driving !== Operating a motor vehicle. If you are behind the wheel of a car and it's running, whether you're in Park or not, you're still operating a motor vehicle.
Consider this: If you get caught with an open alcohol container in your hand, even with your car in park, and you're above the legal drinking limit, you will be arrested. I'd imagine that the same legal philosophy can be applied to texting and driving.
On a more personal note: I can't stand to be around smartphone-operating (i.e. texting, perusing) drivers. They unknowingly drive slower than everyone else because of some kind of limbic reaction in their brains to be "safer" while they do something else that requires their concentration. While doing this, they are playing peekaboo with traffic - up and down, up and down with their eyes, playing russian roulette with those around them in rush hour traffic.
Driving texters may be nice people at every other point in time of the day, but when they're swiping their thumb and reading that "important business deal" text or that must-read Twitter feed while operating a motor vehicle, they are ACTING LIKE TOTAL ASSHOLES .
If you need to communicate instantly and "the text can't wait", then just CALL them instead. At least you'll keep your eyes on the road.
For those of you whining about this overzealous, "dickhead" cop as if he's profiling innocent drivers and crushing libertarian ideals with his nightstick: Be honest with yourself and admit that the overquoted idea, "with freedom comes responsibility" applies here. Admit that texting (or otherwise using the "smart" part of a smartphone) and driving puts other drivers at risk - even at a stop signal. Those texters are operating a potential 3000+ lb. battering ram, and they need to treat their vehicle as such.